If pedal pressure is the only issue a 15/16” master cylinder is the way to go for manual brakes. Otherwise some kind of booster is in order, depending on your vacuum levels.
If stopping power is an issue, you probably have something wrong with your brakes. The stock 73+ disks and drums actually work pretty well if they’re in good shape. Make sure the pads/shoes aren’t contaminated, the drums are properly adjusted, and the factory combination valve is balancing the braking front to rear (it can get out of whack if the brakes are bled improperly).
I would make sure everything is working properly first. If you have 15” rims, you can buy a set of 11.75” caliper brackets and run the later Mopar 11.75” rotors, they came on some of the 77-81 B/R bodies. You can order the brackets from DoctorDiff. Also, if you actually have A-body calipers, you can use the later FJM body calipers. Externally they’re the same, but they have a larger piston bore than the A body calipers. Both those and the A body calipers will work with the 11.75 caliper brackets.
If you’ve got 14” rims you’re stuck without a rim upgrade. And if you’ve got 15” rims the 11.75’s are about as big as you can go.