I have a '40 Ford truck with a 9'' and disc conversion on all four wheels. can I use the booster and master cylinder from a '89 mustang? does it matter that the mustang had drums in the rear and disc in front?
Yes, it matters. Much better off using a later disc/disc booster/master cylinder from a Mustang, Crown Vic/Lincoln TC, F-150, etc.
I would think the dual master cylinder for double disc would have larger reservoirs both front & rear. HRP