the easiest thing to do is to also swap on the master from the mustang. it's matched to those calipers already, and takes all guesswork out. The Mustang is using vaccum assisted brakes too, right?

I'm running Porsche 6-piston brembos on 14" viper rotors on the front of my Dakota, and the front calipers off the 1st and 2nd gen Vipers on 13" rotors are mounted to my rear axle. I switched to the 03/04 Dakota master cylinder because it's the same exact part as the gen 3 and 4 vipers, which had the same size rear calipers, and 4-piston fronts, plus it's a disc/disc master where the truck used to be a disc/drum.

An alternative is to compare the size of the mustang master to the one currently on your car and if close enough, give it a shot. It can get tricky when you mix and match parts from across brands and build a system that was never 'designed' to be together by OEMs. you may want to reduce master size to get the pedal feel you want, but then may run into lack of volume and run out of pedal stroke before maximizing brake effort.

Hydroboost is an option, just depends on how far from stock appearing you're willing to deviate.