70 Jimmy 4x4 driveline upgrade

[k-nuts]

So I’m going to upgrade the drivetrain on my Jimmy. Currently it has a tired leaky 350, a very leaky th350, and leaky 205.
I have opportunity to replace the 350 with a gm crate engine around 400hp. & I’d have the 205 gone through and resealed.

My Main question is the transmission. Rebuild/beef up the th350, replace with new th350 or swap to 700r4 for the overdrive?
(When I swapped the 2 speed for a 200r4 in my 68 firebird it was pretty much just change Xmember and setup tv cable and lockup switch.

Not sure on dimensions or what else is involved in changing to a 700 and if it’s worth it. I looked through posts but a lot are older and apply to 2wd trucks etc, so figured I’d ask again.

This is not a daily driver but an every so often weekend cruise around vehicle. Of course the more I do to it, the more I want to drive it. I just started really messing with it again this past year or 2 after mostly sitting in the garage for over 12 years. I put a hydro boost brake system, brakes, 33” tires, soft top and dash stuff so far. I have 2.5” springs, shocks, and stabilizers waiting to go on so some progress is in the future.

[truckster]

Here are a couple of threads to get you started:

http://www.67-72chevytrucks.com/vboa...ht=700r4+np205

http://www.67-72chevytrucks.com/vboa...ht=700r4+np205

[DeadheadNM]

I’ve got a th350 in my Blazer and a 700r4 in my K20 with 3.73 and 4.10 gears respectively: both with 350 sbc and 33” tires. The Blazer with that driveline/tire combo drives fine to 60-65mph. The k20 with 700r4 will cruise nice at 75mph. There is very little clearance at the front shaft yoke in the k20.

I recommend keeping a th350. It’s not worth the hassle and added expense to swap in the 700r4. If I were to redo my k20 I’d likely make use of a th350 with a gear vendors overdrive. My Blazer has its original drivetrain and my k20 has a correct 350 or I’d have entertained an engine swap.

[k-nuts]

There’s also a semi local low mile (<40k) 2016 Sierra 6.2L Complete pullout w 8L90E and xfer case for 5500.

I know the xfer case won’t work, but this is another option that obviously would require more work but actually Maybe less expensive in the long run from my first plan. Thoughts? I don’t want to experiment to see what fits and what works. I want to know what i need and order it. 😁

[hemi43]

Originally Posted by k-nuts

There’s also a semi local low mile (<40k) 2016 Sierra 6.2L Complete pullout w 8L90E and xfer case for 5500.

I know the xfer case won’t work, but this is another option that obviously would require more work but actually Maybe less expensive in the long run from my first plan. Thoughts? I don’t want to experiment to see what fits and what works. I want to know what i need and order it. ��

Going with the 2016 engine will be too expensive in the long run.Trying to use it's factory computers will be a PITA, so aftermarket is the way to go but it's not cheap. Also, putting in any overdrive will require you to get rid of those 3.08 gears to something like a 4.10 if you haven't done so already. Make sure you check if an adapter is available to go from the 8L90 to the 205 transfer case.
I always tell people looking to swap to a later model engine/transmission to use a 2002 5.3.
2002 was the last year of drive by cable, and the factory computer is easy to hack. The problem is that those engines are getting difficult to find with low miles.

[k-nuts]

Thanks for the all the replies. Just can’t decide which way to go. But good to know about the newer computer systems. I though they all could be easily flashed or whatever it’s called. I don’t want this to be an ongoing upgrade that takes forever to get back in road.

I ran my ‘02 6.0 to 300k before selling my 2500hd. So I know those engines can run forever but rather not gamble on a high mile pull out.

Another option is the EFI version of the GM 350 crate engine I was looking at. Just don’t know if the extra expense is worth it?
I think I would just need fuel pump and return line (so a new tank?)

[truckster]

You might want to watch this before going with that transmission:

https://www.youtube.com/watch?v=QGA44xKGLpU

[k-nuts]

Ya. I have one In my 19 Sierra now. Has a couple jolts when going from reverse, stop and shift into drive. Dealer says “can not duplicate” problem. Because it’s random and not very often.

As far as the Jimmy goes I’m leaning back at the gm crate engine and rebuilding (beefing up) the th350.

[truckster]

Originally Posted by k-nuts

Ya. I have one In my 19 Sierra now. Has a couple jolts when going from reverse, stop and shift into drive. Dealer says “can not duplicate” problem. Because it’s random and not very often.

As far as the Jimmy goes I’m leaning back at the gm crate engine and rebuilding (beefing up) the th350.

But the fact that GM says they're not responsible for it because it's a design flaw... between that and the new logo, I'm not sure I'll ever buy another GM product (except classics, of course).

[hemi43]

Originally Posted by truckster

But the fact that GM says they're not responsible for it because it's a design flaw... between that and the new logo, I'm not sure I'll ever buy another GM product (except classics, of course).

That's why I lease them.

[k-nuts]

Originally Posted by hemi43

That's why I lease them.

But I drive around 25k a year and work out of it. So that’s not an option

Still the nicest truck I’ve had, and haven’t had big issues. To me design is part of the workmanship and shouldn’t be an issue if something doesn’t work right!

[jaydex]

Deadhead I am wondering what you did about driveline angle? Mine is just about done and I keep looking at how steep the driveshaft is. I went with the 350 and a 700 r4 with an np 208. Dropped the trans mount about 3” tilted the rear ended with angle shim as much as I dared and even welded the axle perches on at a different angle. It is a steep angle for sure for such a short shaft. I can’t Rene eat the angle but will recheck.

[DeadheadNM]

Originally Posted by jaydex

Deadhead I am wondering what you did about driveline angle? Mine is just about done and I keep looking at how steep the driveshaft is.

The 700r4 is in my K20, lwb pickup and mounted in the same location as the th350 it replaced. With zero lift, stock height springs there was no need for any driveshaft adjustments that I recall.

Recently a member added a gear vendors overdrive to his K5 and commented on the driveshaft angle and what was done. I’ll dig for his post and edit it onto this post when I find it.

EDIT: see post #10 here http://67-72chevytrucks.com/vboard/s...d.php?t=814872

[jaydex]

Sorry didn't realize yours was a lwb. Mine is in a blazer with a 4" lift seems pretty steep