Help me stop!

-No, a smaller bore in the m/c will probably not cure your problem.It will decrease the effort slightly to produce the same braking force as now, while requiring slightly more travel to do so.Or looked at another way, it will increase the clamping force slightly for the same amount of pedal force. The numbers are small. Going from 1 inch to 15/16 would be 6 or 7 %. This not likely to be significant if your brakes are as lousy as you say.
-As to ceramic pads, Ive never felt the need for them. I have heard that they need to be warmed up before they really bite.Not the best set-up for a streeter.

-When you say you have a Russel prop valve to the rear, just how many valves do you have?
-The proportioning valve has no effect on the front brakes. It simply reduces rear pressure to prevent rear wheel lock-up which has a nasty side-effect.
- Do you also have a metering valve in the front line? That valve looks like a Y-block but has a little rod sticking out of it with a rubber bellows around it.This valve shuts off the complete flow of the brake fluid between 3 and 135 psi. This is so that the rear brake shoes get a little head start.It was only used on disc brake cars for a few years in the late 60s/early 70s. This valve is usually under the battery tray on the rail. Do not ditch it. It is highly unlikely to be the cause of your problem.
-Somewhere close to the m/c should be also a brake warning switch.
-With all the swapping you have been doing with zero results, Im leaning towards, a bad pedal-ratio, the calipers being too small, or not built sturdy enough, or the pads just arent biting.This assumes you have a hard pedal. Just what size are your pistons.
-By 73 we had the combination valve.
-If you have a combination valve, the 73up kind;there is a valve in there for proportioning to the rear and a brake warning sw.
-The switch in the block just turns on the brake warning lite. So,turn on the Ig.key,and pull on the park brake to prove the bulb works.Release the parkbrake to turn off the lite.Works Ok? Next go under the hood, pull that wire off the sw. and short it to ground. Lite on proves that part of the circuit works too.Put wire back on switch to turn lite off. Lite off?All is good,switch is centered.
-If the lite is on before you start, pull wire off sw. Lite goes off, means sw.needs centering.Lite doesnt go off, means park brake stuck on, so go yank wire off it. Come back, put wire back on sw.. Lite goes on means sw.needs centering. lite off means all good. go turn key off and fix park brake sw. put wire back on.Recheck lite still off with key on.
-Centering switch;another post.