has the proportioning valves.
With discs on front and rear you can run WITHOUT a proportioning valve, but need a junction block with the brake light switch (like drum brake cars, which also have no proportioning valve). When you have like brakes front and rear the rear brakes are usually designed to have less "grab" than the fronts, hence no valve needed. On a Jaguar, for example, solid rotors with two piston calipers are on the back, big vented rotors with four piston calipers are on the front (I run a Jag rear axle w/stock calipers, and checked the front out when pulling the rear).
On a custom installation this isn't necessarily so, so most people will run a proportioning valve to ballance out the system. Most people will run a standard GM disc/disc proportioning valve which will give some balance, or you can put an adjustable valve in the rear line. The stock drum valve will work, it just may not let enough pressure go to the rear brakes. The brakes will still operate, just not as well as they could. Proportioning valves limit pressure to 200-400 psi for drum brakes. All I know for sure are AMC application pressure. The Javelin (similar size to Mustang and Camaro, circa 1970-73), Hornet, and even Gremlin used a 200 psi valve, the big cars (Matador and Ambassador, comparable to a late 60s Chevelle and Impala) used a 400 psi valve. Again, those pressures were with drum brakes.
The only way to find out if your pressure valve is set right it to go to a wet parking lot, sprint up to 45-50, then slam on the brakes. If all four lock you're set right. After locking though, make some more runs with less pedal pressure. What you're trying to do is see if there is a point that the rear brakes lock before the fronts. You might try it a bit faster, but make SURE you have plenty of room in that (EMPTY) parking lot! With the weather like it is, I'll add that you want WATER -- not ice! What you're looking for is the rear brakes to lock about the same time as the fronts or only slightly before or after. If they lock to much before the fronts the back end will slide around unless you're perfectly straight. If they lock only SLIGHTLY before the fronts they will give you a warning that the fronts are about to lock and you can pump or let slightly off the pedal. If the fronts lock SLIGHTLY before the rears you're close to maximizing rear brake potential. If they NEVER lock (but the fronts do) you're losing a lot fo braking potential.
The important thing is to make sure the residual pressure valve is out of the master cylinder if using a drum brake master. The reservoir size doesn't matter -- the big reservoir is used because the piston in a caliper doesn't fully retract like the pistons in a drum brake wheel cylinder. So the fluid level gradually decreases. The big reservoir was an attempt to "idiot proof" the front discs -- there's enough fluid in there to safely operate the front brakes for the life of the pads if you never check fluid level. So you can use the drum brake master IF the bore size is correct for the calipers you have. Check the bore size for the vehicle the rear calipers came from. The wrong bore size can affect braking power! Note that later GMs (started in the 90s) have stepped bore master cylinders -- the front and rear portions use different bores. Unless you're sure about the bore, change the master cylinder to one made for disc/disc operation. That's the safest way to go!
Here's a good question -- what's more important, going fast or stopping?? To many people upgrade engines and leave the same old brakes in a car. This is especially dangerous for older sixties and early seventies cars with what could be considered marginal brakes using today's standards.