Their description says:

"Adds dual master cylinder without booster. Designed for all drum applications and disc front, drum rear
need to use the 5912 bracket. needs 0518-WF switch and fitting. Drum Drum does not require ANY valving, no proportioning valve and no in line valves. It sets right below the stock access hole with the 5912 bracket."

(I made the text bold for emphasis.)

 

I know the mustang is a very popular choice, and I speak from a little bit of ignorance (on the 51+ models... my forte is 48-50)..... But what I did was go to the parts store, go through the book and find all the 1" and 1 1/8" dual circuit master cylinders 1969-1972 (FYI dual circuit masters became mandatory in 1968 and for just about everyone that was a 1 year model - and a mostly crappy year too..., avoid designing or building anything brake wise with a 1968 base)

Original for me (1948) was 1 1/16" - can't find that in a 69+, so ya go up or down 1/16".
I forget which way is which, but if you go one way it increases pedal travel (distance) and decreases effort (soft pedal) and if you go the other way it does the opposite. (shorter, hard pedal) - someone who remembers specifics add the right explanation.

So a 1969-1972 econoline kept coming up as drum/drum with built in valves, and the right size. All these aftermarket places want $100+ for the mustang cause it is so popular, but I found a cardone model 10-1485 (that's the econoline spec) worked perfect. (it's 1/4" longer throw, but you'll see why in the next sentence)

I fabbed a adapter plate out of 1/4" scrap (3 bolt holes with countersunk bolts going forward into the crossmember, and 2 bolt holes with countersunk bolts going backward into the master cylinder)...

An hour of fab work and $18 at the local checker and I was done, no need to spend $150+++