Since I'll be swapping out the 7 1/4 for an 8 3/4 in my A body, can I gain more stopping power with reduced rear wheel lock with rear disc brakes?

Locking up indicates you have too much stopping power, either by design, driver over braking, or stopping conditions, which are numerous and variable.
However IMO, only 5%? have the talent to modulate brake pressure enough to take advantage of the better modulation discs can offer, in a perfectly balanced brake system for the situation at hand.
You have not yet shared a reason to justify a disc application.
And, tires stop the car, brakes stop the wheels.

A bodies are known for rear wheel lockup. a smaller wheel cylinder balances it out.

Modern self energizing drum brakes have more stopping power for a given foot pedal pressure than discs. You won't be gaining anything in regards to stopping power all else being equal.

I've fooled around with truing drums and can keep them relatively true with respect to how much track use the cars gets. I'm no road race warrior. Drag racer moving in a different direction. What I have discovered is the high spots on the drum brake surface. I'll drive the car hard in traffic(off track) to help with the uneven wear and then return to the track with seasoned drum shoes. This helps but an inevitable lock up is heading my way. Skill level an issue(yes, I've been driving modern cars too long and unlearned the art of driving w/0 anti lock breaks) but I think multiple minor mechanical issues are at hand, small but accumulatively important. I'm attempting to compete against my son with his late 90's BMW. I've yet to find any info to the set up and driving manner of old iron.This is why I'm here looking for info. Any help is greatly appreciated

There are variables: had a '73 Dart street car (disc/drum) that would tend to lock rear before front if I hammered the pedal - more than the other 7-8 A bodies I've owned.
Drag car would lock fronts before rears - but that had frontrunners & 13x31 slicks, 4 wheel drums (avatar).
Current Valiant (4 discs) doesn't seem to lock anything yet but haven't "pushed" it yet.

Rear brakes do about 30% of the work stopping wheels/tires, tires are the interface @ the road surface.
The urge for rear discs seems largely unnecessary - almost a fashion statement - unless one is using the brakes hard & often, like a road course car.
FWIW, my Dually - disc/drum, when loaded & towing - stops very well - and yes, different animal.
My old Pathfinder on the other hand - 4WDB - could lock all 4.
My Road Runners - one disc front, other all drums - are pretty equal, though I don't drive 'em like I do my Mustang (4WDB)...
My point is that there are a lot of variables, as well as environments/usages, so a blanket statement isn't necessarily accurate.