there is a sigma model (gsi or gsr i think, can't rmember it was 4 years ago, man i have bad memory) that has a rear disc setup that fitted into a gj model perfectly, which was just right for the Commodore v6 conversion that was done in one.
GK GSR was the first Stigma model that had the much stronger Mitsi diff, and all of the scorpions had the same diff (but different rear disc/handbrake setup).
I used to have a couple of GK GSR's and they weren't too bad.