What wheel size are you planning to use?
Obviously if you use GTA Brembos then these will need a 28mm wide disc as this is what they were originally designed for (305x28mm). I dont know anything about the 75 brakes but can you use a disc designed for another Alfa to give you bigger discs, maybe something like the 164's 281x26mm discs? I think also some of the Fiat range use 285x28mm discs
ive tried looking but they have different offsets.... i think 156 GTA early (non radial caliper) discs can be altered to fit? typical, they aint all the same
75's use a 269x22mm disc?