You will not get much of an improvement with the 350TH to 400TH swap. I think the only advantage with the 400 is longevity. It would last longer than a 350. Simply because it has more plate wear potential. Having said that a 350 should stand up good to most street use. I have always wondered if a 350 trans verus a 400 trans has anything to do with the tendancy for a Pontiac to run hotter. Does a 400 trans need more cooling than a 350 trans? The answer is most likely yes.