Charly,
Your front brakes do the majority of the braking, and the 9" MII rotors are nominal at best. You should look at upgrading to what's called a "Big Brake" kit for the MII. Check ECI www.ecihotrodbrakes.com, they make a good one that doesn't widen the front track.

If you use a 4 wheel disc master cylinder, Corvette is most often used, you shouldn't have proportioning or volume problems, but if you're mounting it under the floor you'll want residual check valves.