Depends on which Supra Gearbox your talking about, the Turbo ones can handle a ****load of power, something like 600Hp/650Nm??Probably more...
But they're pretty damn rare, and very expensive. Plus youve got to find a conversion kit and that wont be cheap either, and a decent clutch.
Some people will try to flog off a Normally Aspirated Supra box which would be lucky to be stronger than a T5.
A better option would be to get a T56 from a wreck LS1 Commodore or something.
Either way, your looking at some large amounts of $$$$
From memory Laminges T56 conversion cost $8k all up with a brand new Gearbox??