Hi,
I tried to do this conversion with Fiat 850 breaks on my '64 SEAT 600. The steering pivot are different (smaller). I don't know, if SEAT and Fiat had the same system in those times, but in worst case you must change the whole steerin linkage. So I made two adaptor plates and put disc brakes from an A112. So, better check it before buying parts that you can't use.
regards,
Florian