[YTNUKLR]

All the GT3 boxes are 6-speed "G50s" (even though they are called variants of "G96"/"G97").

The '99+ "G50" (G96/97 trans. supplied in Turbo/GT cars) gearboxes all use a cable instead of a shift rod: this makes it kind of a pain IMHO to use with a vintage 911, AND I personally prefer the direct feel of the rod linkage.

A G50 will require replacement of torsion tube w/ '87-89 911 type, OR you have to shorten the gearbox to fit in your '83. The shortening requires complete disassembly of the transmission. While you're in there you'll probably rebuild it.

A 6-speed will require complete removal of torsion tube section, and you have to shorten the gearbox. Removing the torsion tube significantly weakens the chassis so other reinforcements are requisite.

A G50/03/05 is basically the same as a G50/00/01, except you probably also need to do some machine work on the nosecone to make it fit around the torsion tube in a vintage 911. You could also just replace the nosecone on the /03 w/ the part from an 00/01.

A G50 type gearbox is a good choice if you plan to turbo the car. Generally a 3.6 (~250#-ft. tq.) is OK w/ a 915, unless racing and it gets hot. It will run ~$10k.

Amazingly, the G50 type transmissions basically haven't changed in 25 years.

I'd say, get everything else figured out, install a cooler and lines, etc., and use a 915. If you really want to turbo the car or install huge torque down the road, do a G50 then.