Th350 or th400? Help please

[meatbag]

Hey yall I have a 1976 gmc 1/2 4x4. I'm swapping my 400sbc with a 454 bbc, my transmission started slipping. It originally has the th350. So I'm considering whether to get another 350 or get a th400. So pros and cons? Will the th400 slip right into the place of the th350? Will the th350 hold up to the 454, it will be brand new.

Also it's my plow truck and firewood hauler. Thanks everyone

 

[Matt69olds]

Unless it’s a really stout big block, a 350 will hold up just fine with a few cheap upgrades.

Plan on a full set of bushings. Machine the direct drum piston the equivalent of one steel plate and friction, that allows you to install 5 friction plates where 4 were factory. Dual feed the direct drum, since this is a snow plow truck use a low/reverse roller clutch from a 4L60 trans. It’s a direct bolt in, but the rollers are a little bigger. A aftermarket pressure regulator spring would be a good upgrade. Open up the 2nd and 3rd clutch apply holes in the plate just a little, you want a good solid shift, but not a knock the dash out kinda shift. If the accumulator piston is plastic, replace it with an earlier aluminum part.

 

[Matt69olds]

Read this:

TH350 rebuild tech

www.crankshaftcoalition.com

 

[meatbag]

**** that's awesome reference guide and info. Thanks alot

 

[nvrenuf]

Those 2 transmissions have different spline outputs and use different adapters to bolt to the transfer case. Swapping to a th400 will require a fair amount of changes.