The later W57 - W58 gearboxes are the way to go if you have 300+ hp in mind. Or you can use the T50 nfrom the 4age which is also a good box but doesnt like extreme hp no more than 300 hp.
I ran a t50 for qite a while with 400hp at the wheels but you can break them at will.
I went to the W57 from a supra .... it cost me $350 for the box. You will then need all the other bits and pieces ... bell housing / tailshaft cut and shut etc etc.
You could go skyline 4 piston calipers and disc like me ???
A pic of the w57 gearbox in a manual trans tunnel. It does JUST fit with a few mods in a MK2.... not sure about a mk1. You will need a custom x member.