First, trying to fix a problem with a new master cylinder that's other than the one supplied with the kit is a bad idea.  You need to know why you have an issue first.  A spongy pedal and poor braking sounds like a system that has air in it.  What master are you using now?

Second, I've used Corvette master cylinders before for GM projects where I added rear disc.  On a Ford with parts designed to work with other stuff, no idea, but these are the questions you need to ask:

Is it the same bore size as the specified one?

Are the mounting holes the same spacing as the Ford booster?

Is the diameter of the back of the master that fits into the booster the same size? (if its bigger you're out of luck, if its smaller you can have a sleeve spacer made)

Will the pushrod in the booster work with the depth and diameter of the hole in the master? 

I don't know if anyone can answer these questions.  Likely you would have to buy the master and take those measurements with it and the booster in front of you. 

Welcome to the forum.

Thanks for the info!
It's got a disc/drum style master with 1" bore which is a 1970 power master cylinder. The same master that comes in the MS booster kit. I was told it has dual piston 93 crown vic fronts and I have 91 Thunderbird single piston rears which are 1.79" ( I have a Thunderbird IRS).

I have power bled the whole system multiple times and the pedal still feels weak. I know 99% of the time spongy pedal is air in the system but its highly unlikely there is.

Hopefully someone has tried it and can let me know. If not I'll go with the 92 ranger master.

Thanks!

The only thing I can add to the expert advice already stated is experience but not with the Vette mc. I have on my 66 late model Mustang GT discs front and rear and the 2000 v6 mc. Works great. I would go withe the proven part combination.

On my 68 I am using a stock disk/drum master cylinder for my conversion to 4 wheel disks, it has a 7/8” diameter. When I first set my system I used a 1” diameter MKVII master cylinder and it would not make enough line pressure to give me the desired stopping reassurance that I wanted.
The car has Lincoln MK VII rear disks. Mustang Steve should be able to tell you if the 68’s master cylinder will fit your booster

Thanks for the responses!
I think I'll go with the Ranger MC with Mustang res that MS has recommended in other posts. Don't want to plumb lines on the engine side if at all possible. I'll just fab some new lines and get used to the look of the plastic res.

I switched from manual to a Hydro-Boost system on my 4-wheel discs and used the 95-97 Explorer MC, Raybestos #MC390269, 1.0625" bore, ports are on the fender side. Yes, it has a plastic reservoir.  I am very pleased with this setup.  

Welcome to the forum.............LOTSA knowledge here fer sure!
I just happen to have a durned old Shivel-lay MC on my 66 (disc/drums) BUTT!!........
I have S-10  front calipers AND they are a 'stepped' caliper. A GM invention. Therefore...they need a GM master cylinder.
Getta Ford MC
6sally6

Welcome to the family.

When I performed the Mustang Steve booster conversion, I used 2001 v6 mustang master cylinder.
Absolute day/night difference.
Initially I had Granada discs in front and stock 289 rear drums...using Granada proportioning valve.

This winter I installed 2016 GT brakes front ( went back to 65 spindles) and rear.

Still works great.
As you mentioned, could use a Ranger one if you want ports on fender side.

No one else has mentioned that the gm master cylinder hooks up the opposite of Fords.  Front port to front. Rear port to rear.   I put one on my Galaxie convertible and wound up taking it back off.