If you have an all-drum setup, you need a master cylinder configured for a 4-wheel drums.
If you have discs front/drums rear, then you need a MC that's setup for this configuration.
If you have 4-wheel discs, then you need a MC for 4-wheel discs.
I've never done this, but you can remove the residual pressure valve from the port of a drum brake MC when changing over to discs. --I've just always installed MCs that were setup for the type of brakes I was installing, though.
If you have a power brake booster, MCs with bore sizes over 1" isn't a problem. If you have manual brakes though, it's best to keep the MC bore size between 15/16" to 1" in diameter. Greater than this with manual brakes is going to be tiresome on your leg if you have to make repeated brake applications.
The '67-up 4-wheel drum brake Mustangs have distribution valves. These have a metering circuit to sense if there is a pressure loss (brake failure). If, for instance, the rear brake line ruptured the valve spool inside the valve body would shift to the side of least resistance (the failed side), and stop fluid flow to the rear brakes, but still leaving the front brakes operational.
Factory distribution & proportioning valves are all designed for specific brake system types. As an example; if you had a vehicle with 4-wheel drums and then converted to Granada front discs, you would need to install the factory Granada (disc front/drums rear) proportioning valve. However, if at a later date you converted the rear to discs as well, you would need to change the valve again for one that's configured for 4-wheel discs.
The old Granada disc/drums valve will work with the newly installed rear discs, but it will not work properly because the Granada valve is designed to meter brake fluid at a specific rate to the wheel cylinders of a drum setup, not to the pistons of a disc setup.
Here are some brake setups I've done;
1.
http://www.ultrastang.com/Ultrainfo.asp?Page_ID=15
2.
http://www.ultrastang.com/Ultrainfo.asp?Page_ID=16
3.
http://www.ultrastang.com/Ultrainfo.asp?Page_ID=16
4.
http://www.ultrastang.com/Ultrainfo.asp?Page_ID=17
5.
http://www.ultrastang.com/Ultrainfo.asp?Page_ID=18