SSBC Conversion help

1) Your present MC sounds like it is for a drum/ drum system. It will work with the discs up front, but know that as the disc brake pads wear, the front caliper pistons move in more and more, and they take more and more fluid out of the MC's rear reservoir (which works the front brakes). At some point, you will have to add brake fluid, and then when you go to change pads, you will have to siphon some brake fluid back out to keep it from overflowing and going all over the place. The later disc/drum MC's , with taller reservoirs and a larger rear reservoir, is designed to get around this issue with front discs. (FYI, this draining of the MC reservoir does not occur with drums as things wear, so their reservoirs can be smaller.)
2) With this car sitting for so long, it is unwise to not replace the MC. Brake fluid absorbs water and rusts up the iron parts and so it is just good sense to put in a new MC anyway.
3) The same applies to the rear wheel cylinders.
4) The 3 rubber hoses in the system will rot on the inner liner and fall apart where you cannot see it, and that will cause all sorts of braking havoc. It is wise to renew those.
5) According to some (AJFormS), if you have 10" rear drum brakes, you may be OK with a prop valve. SO do you have 10" or 9" drum brakes? If you try the setup without a prop valve, find a level gravel surface and try braking with varying pedal pressures to see which end locks up first. If the rears lock up easily before the fronts, then you'll need a prop valve. This is all somewhat variable with brake pad and shoe material BTW.