I think you have several things working against you. Heavier car, smaller diameter discs, larger master, manual brakes. I assume you have the correct manual brake linkage which gives more leverage vs the PB linkage.

A smaller master would help. Enough? Not sure. Too small and you may run out of fluid volume or pedal travel. 15/16" is the smallest I'd go. Dr Diff offers one in Aluminum. May be worth a call.