Definitely take an old one apart to clean and change seals. Unless the brake fluid was kept pristine, it likely has internal corrosion which keeps the pistons from sliding to function. Even on a simple drum-drum car where it is just a distribution block w/ imbalance switch, the piston for the switch is often stuck in place so won't give a dash-lamp warning.
Re optimal proportioning, that depends on all brake parts matching the year and configuration for the prop-valve. May also depend on the tires, and unlikely you would have bias-ply today. Even depends on weight distribution, as evidenced by the prop-valve on pickups and vans often having a feeler lever which senses rear sag to adjust the proportioning. Not needed on a sedan, unless regularly hauling bodies in the trunk (mafia). Optimal is that the front tires skid just before the rears, which you can test in an empty parking lot perhaps when wet (need helper to observe). An adjustable proportioning valve for the rear circuit makes that easy to adjust.
A prop-valve is usually for a Front disc - rear drum car. If yours has rear disks (never factory for old Mopars), I don't know if any aftermarket ones could adjust low enough to make the rear disks give proper braking (disks require more fluid pressure than drums). Without a prop-valve, the front-rear ratio comes from the brake parts used and there isn't the selection in caliper sizes like drum wheel cylinders allow. You can realize rear disks via either an aftermarket kit or modified Ford 8.8 rear (Explorer, many posts).