ALL brakes system designs use the wheel cylinder and caliper piston diameters as a fundamental part of the proportioning; it is the main thing used to set proportioning; the prop valve varies it from there.
To the OP: I agree with the comment on the brake shoes; they may look good, but you don't know how hot they have been in the past or it they got soaked in brake fluid or axle fluid; that can effect coefficient of friction of the shoe material and that effects braking all over. Similarly for the rear drums; they need to be turned, IMO.
May have to bite the bullet and turn the drums and get shoes, cheap insurance..
Did you put new pads in the fronts? The pad material relative to the rear shoe material is another item that sets basic brake system F/R proportioning, and who knows what you got off the older car.
I put new Wagner brand OE pads on the front.
Another way to check the booster effect on all of this is just disconnect the vacuum line to it and plug it where it goes into the intake. Bleed off the residual vacuum in the booster with the multiple pumps of the pedal and then go drive and see if the grabbiness is still there or not. That will separate the booster from the rest of the brake system as the issue. (Be a bit cautious 'til you get the feel for the non-boosted brake action.)
So simple, it's genius.. I'll test this way as well to try to isolate the issue.