It's the bore size that counts. The size of the reservoirs makes no difference.
I'm going to go back to your 1st post and say that the OEM Ford front discs aren't really all that hot for a 500hp ride. Heck, no muscle cars of the era (and I drove most all of them) had brakes worth a darn except for a few (4 wheel disc Vettes, Boss 302's, Z-28's). Point is you may still need to upgrade the fronts to bigger and better rotors/calipers even with rear discs. Try your changes now and then decide on rear discs....and then maybe better fronts. I did on my '70 Mach 1 and although it was all quite pricey, the car now stops like an SCCA racer.
If you are using the 70s single piston calipers, the reservoir makes a difference. For sure a drum/drum master is short on volume in the rear chamber to safely (in my mind) operate the big piston front units. I run single piston front (75 Comet), drum rear, disk/drum (76 Maverick) manual master with a 15/16 bore and organic front pads. I have not changed or wore out a set of pads or shoes since a freshened them in 2004. No problem stopping a 3250 lb race car from 90+ MPH on the track, stops fine on the street. Holds fine in the burnout box. To add to that, I bracket race which means I'm on the brakes either riding or punching at 90~95 MPH in the lights when running the finish line. I might look into upgraded disks on the corners if the car got over, say, 100~110 MPH in the 1/8 mile. I'd have no issue running 6.80s,90s or so in the 1/8 on the stock system. Passes - I have no idea in the last 16 years, but between 2003 and 2005 when I was saving ET slips, I tossed out 2200 or so slips.
Also note that most of the braking - 70% or so - is handled by the fronts, so the rear isn't all that busy most times. Running SCCA or some such track racing is different. On the street, meh. If you like 'em, run 'em.
Multi-piston calipers likely don't need much master volume. The dragster weighs 1650 lbs with driver and runs 126 in the 1/8. The braking system is two 4 piston calipers and a bitty 7/8" bore master cylinder. There's more volume in a coke can.