Thomas, this might be of help to you. I was looking for an old article that I had written for the "Fiat 500 Enthusiasts Club" magazine (I do a lot for the mag) when I chanced upon an advert by somebody selling a 'disc brake conversion' for a 500/126. Sadly the advert was for a couple of years ago, but what caught my attention was that he was using "Fiat Punto 55" calipers, and the kit was for fitting to STANDARD 500/126 cars, which means that they must have had 12inch wheels fitted. If you are going to be ]making your own kit, finding a pair of calipers off a "Punto 55", might solve the problem of what to fit that is easy to find replacement parts/pads for.