1972 GMC Farm Find LS Swap

['68 C10]

I've had this truck for almost two years now. Original 350/TH400/AC cab truck that I acquired from a gentleman that used it as a farm truck for several years. The original 350 had been replaced in favor of a crate 350 in the 80s. Needless to say, it has been a bit wheezy since. Still has a noticeable chop at idle but at times I couldn't tell if it was a perfect miss or the cam lol.

When the time came, I ultimately decided to go through with the LS Swap. I obtained a motor/trans/harness PCM for a gentle $900 locally out of an '01 Silverado. I preferred a pre-03 motor to have a DBC setup.

Here's what I'm working with. The truck has some rocker panel issues but other than that, there's really no other trouble spots. The patina is what sold me on the truck.

['68 C10]

Although I really wanted to attempt to retain the stock location AC compressor and mate it to the existing AC unit in the truck, I haven't read very many success stories so I ordered the CPP motor mounts and sheet metal perches for non-AC setups for more adjustability. Also purchased the tubular trans crossmember and trans mount. Down the road I'll delve into getting the AC back with a retrofit unit via Sanden etc etc.
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['68 C10]

Pulled the front end off, got the old lump removed, the perches bolted down, and the new setup stabbed in with the mounts. Aligning the motor took some fiddling, but ultimately got it mated it. The sliders make it really easy to adjust the motor's placement. It's currently sitting with the sliders all the rearward. Stock truck oil pan cleared the crossmember easily.

['68 C10]

For the exhaust manifolds/headers, I decided to go with Speed Engineering swap headers over the initial plan of LS3 stock manifolds. The fitment of them were really good, the flanges are nice and thick. I did use a set of GM manifold gaskets, as the supplied gaskets weren't up to snuff in my opinion.
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['68 C10]

Little bit of an "Doh" moment that was overlooked.

In the process of getting a high pressure PS hose made, I realized we forgot to remove the ground/positive wire ground on the driver side of the block. The seller of the motor had cut the battery cable harness and this was one of the victims. I'm not really looking forward to pulling the motor back up and/or removing the perches and sliders to solve this considering how fiddly it was to get the motor stabbed. Suggestions?
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['68 C10]

Got the fuel system plans sorted for using the stock in-cab tank.

Walbro 255lph inline pump with 3/8" barb to -6AN feed fittings.

Corvette regulator w/ -6AN feed and return fittings

The plan is to utilize the factory 5/16" '72 return on the sending unit, considering the return line size of the regulator is 5/16" as well.

I obtained a returnless fuel rail from the local Pick-A-Part and mated an EFI quick connect to -6AN fitting on it.

Now just need to run the braided lines and that issue is solved.
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[boynton]

did you buy that motor out of Houston area? looks like it took a swim. looking like a fun project!

['68 C10]

I bought it locally, here in the Panhandle where it’s dry the majority of the time. I’ve already placed an order with TSP for lifters, oil pump, gaskets and PRC Stg 2.5 heads. Might as well make some extra HP while it’s apart. Going to pull the heads and pan this wkend and check out the internals to see if the short block is even worth running.
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[boynton]

Originally Posted by '68 C10

I bought it locally, here in the Panhandle where it’s dry the majority of the time. I’ve already placed an order with TSP for lifters, oil pump, gaskets and PRC Stg 2.5 heads. Might as well make some extra HP while it’s apart. Going to pull the heads and pan this wkend and check out the internals to see if the short block is even worth running.
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Sounds like a good plan. Can’t wait to see it come along!

['68 C10]

Started the tear down to assess if there were any issues on the bottom end and the lifters. #8 cylinder did have some rust as well.
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['68 C10]

Several pads of scotchbrite and cleaner/wd40 later, its cleaned up good. Happy to find no pitting or gouges in any of the cylinder walls. Disregard the dark stuff at the end of the cylinder wall, just some leftover debris from cleaning the walls and deck surface.
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['68 C10]

15 of the lifters came out in good shape and I was starting to get a little disappointed I had torn down the top end of the motor...but #16 came out and validated my decision.
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[1971Stepside]

Installing the cam, springs, etc. on mine this Monday. Now I'm worried...I've pulled my valve covers and it is clean so hopefully all is good.

Nice build, I'm watching your progress.

['68 C10]

I doubt you have anything to worry about. Despite the rust, my motor did actually fire up and run, so I wasn't worried about the bottom end much. The top end did need work. I found two of the plugs coated in white on the tip and one plug looked brand new so I know there were some issues. Now all is well though.

['68 C10]

Started off the day by thoroughly cleaning out my garage. I don’t have a pic of the madness, but just let me say that it’s crazy how many boxes and parts you can amass throughout the course of a project.

As far as progress goes, I got to work on getting the radiator in it’s new home. I was initially going to use a stock 99-07 truck radiator with LS1 fans, but decided to go the Champion route with the Fans already installed. I’ll just have to modify the electrical connections on the end of the Champion fans to mate to the Nelson Performance Fan harness. I needed up modifying the factory mounts by shortening them. The radiator is about 2 inches less wide than the factory radiator but it still fits on the factory lower supports.
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['68 C10]

In order to route my exhaust through the Porterbuilt crossmember ports, I discovered I’d have to change trans crossmembers as the CPP one sits right in the middle of where the exhaust would be routed to tuck up in the frame. The solution was to order a Dirty Dingo Trans crossmember. The humps will provide enough exhaust clearance and still have space between the member and the cab floor. Took some fiddling with the parking brake cables to mount it. I ended up installing it with the cables above it. I’m sure it’ll rub over time if the brake gets utilized often, but I hardly use mine. Fit and finish of the member is great, I’d recommend it to anyone.

['68 C10]

Worked on the battery tray as the day of putting the front end back on grows closer and closer. I cut of a couple of portions, including the AC brackets, the lipped portion on the side, and the extra metal around the top that wasn’t needed. I then added a piece of angled metal to the upper portion in order to mount it to a factory bolt on the upper driver side. Then just drilled the side holes into the bracket and into the core support. Came out better than expected, and also didn’t have to relocate the horn.

Front end goes on tomorrow, then it’s plumb the cooling system and mount the overflow tank. Driving is near!!!
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[boynton]

Nice work on the battery tray. I also really like that dirt dingo trans mount. I have the cpp on my truck but that would have probably helped me a bit!

[c104life]

What was the part # of the champion radiator you ordered for your truck

['68 C10]

CC162 if I remember correctly.
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['68 C10]

Got the front clip back on. Looks like I’m going to end up doing drop shackles and lowering spindles as well, doesn’t quite sit as low as I would’ve liked.
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['68 C10]

Started the task on plumbing the cooling hoses etc to the radiator. After several trips to the parts stores I’ve got a makeshift set up until I can get a better reservoir, as the one I intended on using had a small leaking crack. Ultimately it’s fine because I wanted to use a different reservoir other than the bulky factory NBS truck one. So for now, I have hoses straight from the firewall to the water pump w/o the “T” in line. In turn I routed the steam hose from the TB to the 1/4” nipple below the cap.

Filled the system and heard the familiar sound of liquid hitting the ground.....and a lot of liquid. After fretting on it possibly being the head gaskets, I discovered the coolant crossover plates had no seals or orings. Quick trip again to the store alleviated the leaking fluids. Got the intake back on and all the hoses connected again.

Now on to wiring the fans and we will be ready for the drive!
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['68 C10]

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['68 C10]

Got the fans wired. I have a Nelson Performance harness that I’ll be using in the future but for now I just wired them to an unused ignition spot on the factory fuse block.
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['68 C10]

Ran into a debacle the last few days.

I took the truck out for it’s initial shakedown run and when I returned I couldn’t help but notice #4 cylinder didn’t have the same color on the primariy as the other 7. It still looked new, and definitely not the “bronzed” coloring that stainless turns when it’s been heat cycled. This started the troubleshooting.

Swapped coils - no change
Swapped plug wires - no change
Spark test - spark confirmed

I ended up doing a compression check on cylinders #2 and #4 for comparison......

#2 - 160psi
#4 - 165psi

So now I can breathe easy knowing it’s either spark or fuel.
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