Not sure why you would want to do this. Anyhow, if you are talking about the newest Supra Turbo [MKIV 93-98]. Then those are 6 speed (V160 Getrag), yes they are plenty strong and can handle upto 700HP easily in stock form. But they will cost you a fortune (over a grand even at junkyard if you can find one). A W58 5 speed from a non turbo will be cheaper (still expensive) but likely give up at around 400HP. A better bet will be a R154 from a MKIII turbo (87-92). They are pretty strong and are much easier to come by.
But again, why would you want to do this? I am not a fan of mixing, keep a mopar to mopar and let a toyota be a toyota. The funny thing is that there are a few Supra drag racer out there who are using 700R4 GM auto tranny at the strip. So what does that tell you?
