They can fit, however only on the cars which have 'small calipers'. The 1750/2000 GTV and spider had bigger ATE brake calipers to accomodate for the bigger engine and the spacing for the caliper bolts is different. The 75 calipers strangely have the same caliper bolt spacing as the small ATE caliper cars (1300GTJ, 1600 GTV, 1600 GTJ) so will only fit those uprights. We race a 65 Sprint GT and have experimented with many different types of calipers and their respective up rights. The 75 caliper has no power advantage over the standard 2000 GTV calipers, the only advantage being weight as they are Aluminium Brembo's. The best way of improving brake power for normal use would be to fit new 2000 GTV calipers, which are available on exchange from www.alfaholics.com
We also use Ferodo DS3000 pads in our race car, which are by far and away the most superior race pad available. Ferodo have made a fast road/trackday pad based around the DS3000 called the DS2500 (also stocked by alfaholics). They have absolutely no fade and do not need warming up like most trachday pads, so are fine for road use.
Another problem with 105 brakes is the balance, with the standard Alfa brake proportioner fitted (under the car under the rear seat) there is far too much rear bias and the rear brakes can stick turning into corners... never a good thing on the track! For trackdays, i would highly recommend fitting an AP screw type proportioning valve and throwing the Alfa unit in the bin! P.S. the Tilton handle type adjusters are not fine enough to use.