I followed what others have done on 155.org. There is a CAD drawing for some adaptor brackets to use with the original M14 Alfa caliper bolts and then some standard M12 bolts to fix to the standard hub.

I think the calipers are bigger (longer) but as you say, thinner than the GTA. I needed to use 15mm spacers to clear the caliper but 10mm would have done it too. This depends on the wheel as some on 155.org have managed with no spacers....depends on the wheel offset.

wrinx