72 K20 350/350 project

[Craig W]

Hi there,
I have as my title states a 72 K20 with a 350 and turbo 350. I lost the rear axle due to neglect by the previous owner. In installing a corporate 14 bolt from a 77. My next goal is to do something about the tranny. My end game is a bullet proof well rounded truck. I know you all have opinions about what to what. My transfer case is a np205(by my best research). What do you guys think? 700, 400 keep the 350? Im not rich so economy is a factor, but lets make this an awesome ride.

Craig

[jeremy zander]

Welcome,,72 3/4 4x4 is an awesome ride indeed..Depending on how you are going to use your truck will determine what tranny to use...I redid all my original drivetrain...It is a great driver..however an overdrive transmission (700r4) would be great for the highway...Build it depending on your use for the truck...good luck and great choice in trucks...

[special-K]

Welcome to the site and the wonderful world of K/20s!! Opinions are like GM trucks,everyone has one...and those who don't really don't matter

It's great to stay with the original NP205 because...they're original and as tough as they get. I have never felt a reason to upgrade to a TH400. The TH350/NP205 combo has been bulletproof in every form of 4wd I've owned. GM got it right with the TH350. Nothing against the TH400 (except they rob HP) and if all I had was a TH400/NP205 combo laying around I'd put it right in. I just wouldn't go jumping through any hoops (or spend a bunch of money) to swap one in place of a TH350. These NP205s don't mate up,you know. I would either stay with what you have and take it easy on the freeway or do a 700r4 swap.To stay with your NP205 will be an expensive proposition due to a conversion or adapters will be required. The NP208s seem to stand up to whatever is thrown at them just fine. Figure that's the transfer case in just about every CUCV K/30. You can find 700r4/NP208 combos fairly cheap. The ultimate would be a shaft conversion to mate a 700r4 to theNP205 or find a late NP205 with the figure 8 mating pattern. The Ultimate ultimate (that just screams expensive,doesn't it?) would be to use a 4L80E. So,in my opinion you should use the TH350/NP205 that's in there unless you have a TH400/NP205 combo laying around,swap in a 700r4/NP208,700r4 adapted to NP205,700r4/late NP205,or some configuration using a 4L80E. That's my opinion.I can't help it if I'm limited and narrow minded.

[Craig W]

Thanks Jeremy, I am looking for highway travel. I live in the back 40 so I wanted a fire road truck that I could go on long hauls in. Do you know if I need to cut my floorboards to compensate for the longer transmission? I'm concerned that the 4x shifter will need to be moved.

[Craig W]

Thank you Special K. I tend to agree with you about originals. I feel a bit like a bastard if I take the original drive train out for no good reason, this truck is all matching and was owned by 1 other guy before me. I wouldn't even had changed the axle but for it failing on the way to a gas station 3 frickin miles from my house! I hear the passenger rear wheel whining, I get out and my axle shaft was hanging out of the axle tubing by 3 feet! That was a sad day for me and the hounds. Not to mention for poor Abigail (my K20). With what you say and my pocket in mind I'll likely keep the 350 for now and look for a 700 to rebuild. By the way, how bad is this combo on gas?

[Bert W]

Hello Craig! I just wanted to say welcome to the board. Sorry I can't help with the K20 much. My truck only has a differential in the back and no transfer case.

[67swb72klb]

the 700 will pick u up a few miles most on here move the engine foward a couple inches and then no issues with driveshafts ect and anyways where is the pics ?? wecome to the board

[special-K]

I get 12.5 in my '72 GMC K2500 350/TH350. I do 60-65 on the interstate and prefer the 2-lanes on all but long distance.
Yeah,the O/D trans is longer but the K-model engines are set back. So,you can move it all forward to the next engine tower holes forward (I6 position). See where your distributor is now? You'll like things moved forward better.

[superchevy]

I guess I have "the ultimate ultimate", lol, in my K25 with the 4L80e/np205 combo behind a 468. If that is a trans/t-case combo you are considering, you can see what I did in my build thread in my sig.
Definitely move the engine/trans/t-case forward if you can when doing a 4L80swap...I should have, but didn't, and I had to modify the high-hump trans tunnel for a lower/longer t-case shifter throw.

[Stocker]

Originally Posted by Craig W

By the way, how bad is this combo on gas?

Not good, though a lot depends on how you use it. The only time I accurately tracked fuel mileage was on a long (~3500 miles) trip several years ago, and it averaged 11.5 mpg (truck in my sig with 4;10 gears). Typical driving for me is closer to 8 mpg, and much worse in winter if I let it warm up several minutes to thaw the windshield before driving a mile to work....

Welcome to the forums, you have found the very best Chevy / GMC truck board in existence! Now how about some pics of your K20?

[mrein3]

Originally Posted by Craig W

Thank you Special K. I tend to agree with you about originals. I feel a bit like a bastard if I take the original drive train out for no good reason, this truck is all matching and was owned by 1 other guy before me. I wouldn't even had changed the axle but for it failing on the way to a gas station 3 frickin miles from my house! I hear the passenger rear wheel whining, I get out and my axle shaft was hanging out of the axle tubing by 3 feet! That was a sad day for me and the hounds. Not to mention for poor Abigail (my K20). With what you say and my pocket in mind I'll likely keep the 350 for now and look for a 700 to rebuild. By the way, how bad is this combo on gas?

You probably already know this but for those folks who don't, make sure you get the right gears in your new rear end. The HAVE to match the front in a four wheel drive truck - unless you want one set skidding or worse something in the driveline breaking.

The very best I've ever done gas mileage wise is 13.5. On the highway. With a tail wind. 350/350/3.73 gears. Mine is a half ton so it is a little lighter than you K20 guys. I usually get 8-10 depending on if I'm pulling my boat or not. In the winter I get 3-5 but that is only because it doesn't get over 20mph pushing snow around my driveway and driving across the lake to and from my fish house.

Oh and I wish I had a 700r4. I don't mind the crappy mileage on the few trips I take on the highway but I HATE the engine spinning at 3300 rpms and all the noise that comes with it.