I got a 68 Belvedere Station with non-power drums on all 4 corners.
Poor braking power at present. Car just bought.

First of all will replace the Fluid and bleed the system. If this doesn't help I will add new Shoes and wheel cylinders on all 4 corners.
I was wondering if I will replace the Master Cylinder if I can improve
the braking force with another Master Cylinder? OEM it is a 1' unit,
aren't there 15/16 or smaller diameter MC's around? Anyone mounted a
15/16 Disc/Drum Master????

May I use smaller diameter wheel cylinders (A-Body for example) on the rear brakes for improving power on the front?

YOU CANT use a disk drum m/c

Yes there's either a 7/8 or a 15/16 avail (one of the two) & check that the inside recess in the M/C piston is NOT smooth like the inside of a sewing thimble but has a wide shallow groove for the neoprene retainer on the pushrod to lock into as manual brakes need that groove and a disc/drum M/C will work excellent, just cant use the RPV if it has one & it would b under the brass seat in the outlet port next to the firewall (the disc half) & check by sticking a drill bit into it (brass cone) & it's just a piece of neoprene and a spring that functions as a check valve & you'll feel it (the spring tension) if it is there & if so thread a screw into the brass cone & lever it out w 2 screwdrivers then toss the neoprene/spring & reinstall the brass seat. The #1 thing that will help you is a set of aggressive shoes with a proper bed in procedure and arcing the shoes to fit your drums if your brake shop has that machine and yes smaller wheel cyls on the rear to add more bias to the fronts