Yes, rear cylinders are new. Installed them when I narrowed the rear axle.
If they were old/unkwown age, I could've bought smaller ones.

I've got an adj.prop valve somewhere, but for some reason I don't like to 'tone down' the rear brakes to match the fronts, when I'm still feeling the fronts should be able to brake better.

Besides upgrading the brake pads again, I have a few other options to improve the fronts, but at all would mean ending up with smaller piston-surface which would offset the brake bias even more, so I will for sure need to go smaller on the rear cylinders aswell.
The current brake system on the front is very heavy because of the '73 fullsize Chrysler parts (caliper & rotor). I wouldn't mind shedding some weight there and go to a lightweight Wilwood system like I have on another car. Been thinking of just swapping it all over as the Wilwood system are brakes way better on the other car.
The swap would be a pretty much no-cost operation so it's very tempting.