I used a HiSpec 300 x 22mm off the shelf disc and one of their universal disc bells to move the caliper and disc as far inboard as possible to give maximum wheel clearance.
I think I did skim a little off of the aluminium hub but can't remember exactly now.
On the rear I used a standard Alfa 156 (front 285 x 22mm) vented brake disc as caliper clearance wasn't an issue with the Gp4 rear wheel.