Replacing th400 w/ th350

I have a 1977 Heavy Half ton C10 with a 454 and a TH400. The engine is still strong but the transmission is lacking. I may come into another old chevy truck that has a th350 in it. How well would that th350 hold up against the big block? Not saying I have a monster engine, but I just don't want to tear a tranny because I am putting too much power through it. Thanks.

Get the 400 fixed.

Ah, I figured that's what I would be told. It's not that it's broken per say, it has just seen much better days. Rebuild kits don't seem to bad for them, however, getting them installed is what seems expensive. When buying a rebuild kit, would you recommend getting one with a shift kit?

A kit that causes less hesitation between gears (faster shifts) may be less smooth for your passengers, but also will put less strain on the wear components of the trans so it can last longer. A large external trans cooler, beefed up clutches and other internal parts plus a heavy duty ATF like Redline D4 all can contribute to a stronger TH350.

Ask around for good local trans techs who have experience with your proposed swap for specific advice.

See if they agree you can get away with improvements that don't require total dissassembly of your "new" transmission.

Frequency of trans service and your own driving habits are also major factors in how well the new "marriage" works.