11 3/4 brakes

If your rears still lock first there are two things you can change that will affect the balance. First, 73-75 A-bodies used calipers with smaller pistons. Switching to the large piston calipers (76 A, later B, F, R etc...) will increase front braking. If that doesn't solve the problem, look for rear wheel cylinders with smaller diameter pistons. I think I've seen some 13/16" or even 3/4" wheel cylinders out there somewhere.