use all th400 parts and save the money you would be wasting
The trick is to start with a turbo 400 case, then assemble it with the parts that fit in the case. This is the most durable 3 speed trans that GM makes.
A 350 is best left to the hot rod / car show / show car wana bees
However, for thousands of dollars, there are several trans shops that will make a 350 as strong as a stock $200 dollar 400.
Bruce