If the rears are locking up first under panic stop conditions then you either need more front brake bias, or less rear.

To get more front bias you can go with higher friction pads, go to larger rotors, use calipers with larger pistons, etc. (I assume you have 11.75 rotors with 2.75 inch calipers? If not, that would be the first thing to change.)

The reverse works for the rear. Smaller wheel cylinders, smaller drums, etc.

Weight distribution and ride height also play a part but those things are a little harder to adjust.

"New cars have no combo valve anymore, just well balanced /engineered piston /rotor sizes teemed with ABS that only kicks in on slick surfaces."

You are talking about 4 wheel disc brakes.

The OP has a disc/drum system.

I'm not a brake expert, but as far as I know, discs and drums don't react the same, so a combination valve (with a hold-off function) is beneficial.

did you replace your rear shoes? Do you have them installed corretly? I know for a fact that if you have the leading and trailing shoes backwards the rear brakes will lock. Thats why I don't drink when doing brakes, lol.

make certain that park brake lever on shoe is fully retracted and not being held by faulty cables.