Not the braking I want

BTW, do you know WHICH GM calipers that you have or the piston diameter? If you got the smaller one, which have 2.38" diameter pistons, then that is smaller than the Mopar ones, and will reduce front brake torque for a given pedal pressure. They will put about 18% less pressure on the pads than the stock A body Mopar calipers with their 2.6" pistons.

Knowing the rear brake drum diameter and shoe width would help too to get some idea if the front pistons are too small.