Power Disk as Touchy as Power Drum

If the power booster is factory original, then it should have the correct pedal in it. Take a look under the dash to make sure. THe power pedal will be hung high up in the hanger close to the firewall, not in the center. Does this car has auto or 4 speed?

Well, thats correct. THe only thing left is the booster is wrong (to much assist-diaphram too large for aplication) but seems unlikely. Or the master cyl is wrong-and I get confused on this. I always thought that the smaller bore = higher pressure less volume, and a larger bore = lower pressure more volume (of brake fluid moved) with the same pedal travel on each. But others have said it's just the opposite.
Ifthe master cyl is the correct unit, then the only thing I can sugest is a 4 way proportioning valve (4 port) but that seems like a last resort. Hate to suggest that so soon.
If you pull the master cyl nuts off, make sure that the rod isn't pressing against the master cyl ram in the back. THere shoul be just the smallest gap. Just short of touching when the mater cyl is bolted up tight. If it's pressing the ram in even slightly, that will bring in the front brakes much sooner. And you will loose a slight amount of trael for the back brakes. CHeck that stuff out and lets see where you are.

Is your car a 1967, or maybe has had the booster replaced with a 1967 unit?

Ford had a problem with the power boosters on the 1967 Mustang, about them being really sensitive. They had a recall, called a "campaign" back then, on these. When I worked at a dealership they still had about a half-dozen of these boosters sitting on the shelf, which got scrapped when the dealership moved.

My 1967 power discs were very touchy when I first got the car. Almost put me through the windshield. Had the booster rebuilt locally and had no further problems.

I had a 66 Monterey a while back that had rediculously touchy brakes, and after a while of playing around, I figured out the booster had a torn diaphram that sealed up when you pushed the brakes. It was small enough that I didnt notice the vacuum leak hissing and didnt really affect the idle much. The brakes were a little hard at first, so naturally you started to apply more pressure to the pedal, at which point the diaphram started to seal, and suddenly you had more assist. The booster on this particular car was a double style (small and deep), and I think the Mustang uses a single large diaphram, so this may or may not be relavent.