Are you sure that you replaced the MC with the same model that was on it before? Disc pads ride very, very lightly on the rotors, but they do tend to build up heat in the calipers. Everything you've said points to the buildup of pressure due to heat, and the MC not allowing the excess pressure to bleed back. Seems to me that an easy test might be to drive it until it starts dragging, then reach down with your toe and pull back on the brake pedal, removing ALL pressure from the pushrod. You did not mention a power booster, so we assume manual brakes, and that your MC is designated for manual brakes, right? Also, in a '32 we're talking below the floor mounting for the MC, right?