Personally, I'd gut the combo valve an then look at a bigger MC. Do some hard braking and see if you need an adjustable valve - if your backs start locking up, you will.
I have jb8 brakes up front - the big Bendix ones which have the same piston area as the 04 fronts I pulled the rear from - so I went w/ an 04 master cyl. Balanced from the factory. And stops well, but was a tight fit...
The van mastercyls are an easier fit, very slightly smaller brake bore (1.3" vs 1.4" I believe) and may be appropriate for your fronts. If you want to come take a look at mine and see what it took to fit that giant 04 in come have a look.
I have to change out the fuel pump tomorrow night then can take it out for some brake checks.
I like this a lot i can get the master for 75 dollars. I probably would just get the adjustable valve so I can really dial it In.

