Fit a Toyota Supra twin turbo engine to GS 300 mk 1?
Presumably it would take up the same amount of room in the engine bay but would the gearbox/ torque converter need uprating?

Not sure on the technical bits and bats of the the supra twin turbo engine, but the Aristo (Japanese version of the GS300) has a twin turbo model which I guess is pretty similar.

The twin turbo supra engine, the 2JZ-GTE, was first used in the JZS147 Aristo (MK1 GS300) so yes you would be able to do a transplant. I'm not sure what gearbox the GS300 has in it but the Aristo has a A340E, they all use the same model number MK3 Supra, MK4 Supra, GS, Aristo, Soarer (I think) and more I imagine. But are slightly different depending on what engine they are bolted to, think the splines are different.