73-87 Chevy _ GMC Trucks > Engine/Drivetrain

TH400 swap opportunity?

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Hey everyone, long story short I bought a used 350 to drop in my truck after mine blew. This new 350 also happens to be mated to a th400 and the guy wanted to just sell the whole package. It came from his daily driver '75 c20 that he just pulled the engine and trans to drop in a ls 6.0/4l80 setup. Supposedly the th400 has less miles than my truck's th350, and they are most likely in similar condition.

Would it not be a bad idea to just leave the th400 on the engine and drop it all in? Will I need a new driveshaft as well?

I would,but you'll at least need a 400 input for your driveshaft or maybe a whole new shaft depending on length differences

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Be mindful that the TH400 kickdown on squarebody trucks is an electrical switch on the go pedal under the dash.

I'm considering if it's worth getting a new driveshaft because I'm fairly certain the two transmissions are difference lengths. And regarding the TH400 electrical switch, is it simple to bolt up?

I also do not know for sure if I can mate the old TH350 to the new 350 without a new flex plate? Or can I just bolt it up?

the TH400 is certainly step up in many ways, but I would consider the benefits vs the costs.

A TH350 is plenty good for most trucks, so a couple of questions:

Is your use of the truck in need of a stronger trans?
Do you race?
Pull out stumps?

If none of the above, I would keep my life simple and sell the TH400.


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