Possibly, possibly not, but thoughts are: why chance it?

The size of the reservoir is unimportant, provided that it has enough volume to move the caliper pistons without running dry.  I'd rather not try and figure all that out, so to me its better to just run a master cylinder designed for a disc/disc setup. 

The other potential problem is that discs and drums operate at radically different pressures.  Sometimes the master cylinders have valves that reduce the pressure to the drum brakes.  An adjustable proportioning valve cannot increase the pressure from the master cylinder; all it does it reduce it to avoid premature rear brake lockup.  So its possible that a disc/drum master simply isn't capable of providing enough pressure to get proper performance from the rear brakes.

All in, master cylinders are pretty cheap.  I'd just swap to the 2000 V6 Mustang master or something else designed for a disc/disc setup. 

True74yamaha wrote:

The reason why I said you need to change the resivour is because the 2000 disc disc master cylinders resivour will not clear some stock mustang hoods.

That statement is incorrect.  If you use the 2000 v6 master, it bolts right on, with or without the MS booster.

You can use the disc/drum mc (Pb-6.1) with disc/disc system.  The Pb-6.2 will do a better job, though. It is the 2000 v6 unit. It does have metric ports in the engine side.