FWD cars generally have a diagonally split system so the proportioning would likely be incorrect. I guess you could get away with it but that's the reason not to use a FWD master cylinder on a front/rear split car.
I`ve got 4 wheel disc brakes on my 68 fastback. used the stock splitter, and an 86 diplomat- Chrysler alum master cyl. 7/8 bore, works perfect . (willwoods on the front)
