ive gone into this topic in great detail
the bottom line is becuase the TH350 doesnt have the kind of aftermarket support necessary to be put into a supra
not to mention, building a TH350 to hold the power a TH400 will costs nearly double
however, they are very similar transmissions, with the th350 being slightly weaker, the th400 has a 2.48 1st, and the th350 has a 2.52 1st
th350 takes about 20 less hp to spin, and it weighs nearly 30 lbs less
but like i said in the beginning... building a th350 to hold 1000 crank horsepower will cost you one damn pretty penny (and the T-Brakes for it cost more then a built th400)
for example... the ATI Ultra Turbo th400 costs $1779 and is built to handle over 1000hp
the ATI Ultra turbo th350 costs $3147 and is biult to handle over 1000hp
^like that price difference?
its like putting 1000hp in a civic... its still a civic