Big Block Conversion Advice?

[Jsposito]

From reading a handful of threads on here, there seems to be varying experiences with swapping a big block into a 67-72 4x4 truck. I was curious to hear of any fresh perspectives, as I'm planning on pulling the original 402 out of my C20 and putting it into my K20 next week. Any advice from those who have been through this project would be much appreciated! Thanks for the help!

[Jsposito]

ttt

Hi all- Does anyone have a big block in their truck?

[kwmech]

How many places you gonna post this?

[Jsposito]

As many as possible to get a bit if help! Thanks for the advice. I appreciate the insight.

[jlk]

Did you mean the K20 had a small block and you're swapping in the big block? Well, the front end will sag because the big block is quite a bit heavier than a small block so you will probably need a pair of add-a-leafs at the least.

Does the big block have headers or manifolds? You may have header clearance issues. You'll probably have to have new head pipes. What year is the C20? Some chevys have the alternator on opposite sides so you'll need an alternator extension wire harness. Your local parts house should have one. The radiator hoses may not align or be different sizes.

That's all I can think of off the top of my head. It's do-able. Good luck!

[67swb72klb]

might have to clearance the firewall they are tight on a 4x4
you can try using the search and turn up some kind of more info

[Jsposito]

Yes, the k20 does have a small block now and I'm putting in a big block. Thanks for the help. I'll post pics when it's complete.

[Bishops Trucks]

Jsposito, I have a K10 that I bought years ago. It was an original 350 with a 350 trans. When I bought it, it had a 396 in it. The springs won't sag. I had no issues with the big block in there except for a little firewall clearance for the distributor. You can get a distributor that is low profile or smaller diameter that will cure that issue. Since it's going in a K20, you will have no suspension issues what so ever with the added weight. Have fun with the swap.

[Yukon Jack]

Have you seen this post yet? It has a lot of good info that I used in swapping in a BBC in my 69 K20.

[Jsposito]

Thanks for all the advice!

[Gobuffalo]

You can mount a big block with all small block stands and mounts. You need to use headers and notch the left side stand. You should use polyurethane motor mounts. You will need to grind off the bottom corners of the left side mount to clear the bump in the block for the oil galley that runs down the left side just above the pan rail. Re-drill the holes in the top and bottom of the frame 2" forward.
If you want the exact bolt hole locations, look at the top of a two wheel drive frame and copy that.
I've done this to my 70 4X4 Suburban and I run a 496 with no problems.
The distance from the back of the block where the trans mounts to the center of the motor mounts is the same BBC or SBC.

[Jsposito]

She's in and runnin!

[snj8198]

looks like you did a nice job. is the cross member located in the original rearward small block holes or the forward ones?

[bL.A.zer90045]

damnit,you forgot the vaseline on top of the air cleaner! NOW THATS TIGHT!!! looks good,glad you goterin

[Jsposito]

@bL.A.zer90045 Ha, ya the I could use a bit lower profile air cleaner...just barely fits!

@snj8198 Thanks, and yes I did keep the cross member in the factory rearward small block location.

[USAF]

I'm about to swap a BB I my 72 K20. Did you have to use the forward motor mount holes or drill new holes?

[kwmech]

If you have the correct short pulleys and brackets, use the front holes. It will help with valve cover and HEI clearance.

[USAF]

Originally Posted by kwmech

If you have the correct short pulleys and brackets, use the front holes. It will help with valve cover and HEI clearance.

Thanks KWMECH. I'm running an electric water pump and elec fans so that shouldn't be an issue. Only running the power steering pump and alternator then. But with using the front holes will this pull the engine too far forward to use existing mounting for the transfer case and manual transmission?

[kwmech]

Sometimes there are matching holes, but you may have to drill and have the d-lines redone for the 1-1/4 or 1-1/2 difference,

[GSP7]

Big Block Conversion Advice?

should be a cake walk

I had a original 1969 CST/396, turbo 400 shortbed chevy...

1967-1972 C/K pickup truck powertrain details

396 cu in (6.5 L) V8 Chevrolet big-block engine (Mark IV) 1968-1970 4-bbl 325 hp @ 4800 RPM 410 lb-ft @ 3200 RPM
402 cu in (6.6 L) V8 Chevrolet big-block engine (Mark IV) 1971-1972 4-bbl 300 hp @ 4800 RPM 340 lb-ft @ 3200 RPM Marketed as 400 cubic inches; same as "Turbo-Jet 400" in full-size Chevrolet

[USAF]

Originally Posted by GSP7

should be a cake walk

I had a original 1969 CST/396, turbo 400 shortbed chevy...

Posted via Mobile Device

[kwmech]

Originally Posted by GSP7

should be a cake walk

I had a original 1969 CST/396, turbo 400 shortbed chevy...

Which sat in the same front hole position as mentioned. I used to have one of those also, it was a lot of fun to drive...only I had a hopped up 454 in mine