would a supra turbo fit on an sc3, just as well as any other? and if so, how much of a gain would that be?

 

Nope...
Exhaust port spacing and bolt pattern is slightly different on the head from the Supra TT.

You wouldn't want the Supra's stock twins on your SC300, anyways, they're too small.

Do a search on turbo kits. Toyomoto and others are popular choices and make more power than any stock Supra TT.