from what I understand, the physical gears in a TH400 a huge compared to those in a TH350, and since there's more rotating mass, it takes more power to turn 'em. the gears in a th350 supposedly take 15hp to turn, while the gears in a th400 take around 40hp. the two trannies weigh almost the same - 130lb for the 350, 135lb for the 400. there's a tranny shop by my house that'll put the 350 gears in a 400 while retaining the 400 case, valve body, etc. it costs $400. But you get a strong tranny with more real wheel power. personally, i myself am going the th350 behind a big block route.