Brake pedal is really stiff

Re MC sizes. I just checked that recently for my cars. rockauto.com shows a 1"D bore MC for both my 65 C & A bodies, whether manual or power brakes. A bit strange because I recall reading that A-body power brakes should have a smaller MC bore. While that sounds opposite, A-bodies have a lever in the stand-off brackets that decreases the force on the MC.

I was concerned because I used an MC from a 99 Breeze in my 2 A-bodies, with a 7/8"D bore and was worried about too much pedal travel, i.e. maybe bottoming out on the floor. I didn't measure, but maybe 1-2" pedal travel in my Dart w/ power assist, which feels fine. I worried after I investigated an ebay ad for a new alum MC for a Volare w/ manual brakes for $25. Turns out those have 1.125" bore, which is much larger than mine. Indeed, not sure how that works since all Volare's were front disks. Volare's must have had large pistons in the calipers to match.

Most people would consider a stiff pedal to be great, assuming it doesn't take too much force, which sounds OK in your current configuration. Another way to get a little more motion like you desire without changing MC or wheel cylinders is to use DOT 5 (silicone) fluid, since it is more compressible (why one can't use w/ ABS). It eliminates rust in the brake system and won't eat your paint if it leaks, but does cost more.