‘66 Barracuda Brakes Suck — Proportioning Valve Maybe?

I don't have many more ideas right now.
To test the pressures would require access to a lift and at least one high pressure gage. Gage set used for testing automatic transmission pressure could be adapted. Not many shops do that, never mind checking brake pressures. But if you know of one, or someone who, that would be a great way to prove or disprove whether the problem is in the hydaulics.

Could try going over the surface of the drums and disks with a block and sandpaper or emery. And try different linings.
I'll say I found old Cycleweld Mopar pads were not very grippy at all - especially on initial application. Previous to them I had used a Raybestos lined pad that was really good, but now is long discontinued. I'm currently using Porterfield's street lining for both the pads and the shoes. They seem to have about the right balance, grip but not too grabby. But trying those Porterfields would be an expensive experiment just to see if the linings would make a difference.