Use any 73 disk brake A-body Mopar for parts look up. Someone went to a lot of trouble to get the upper control arm, spindle, rotor, and caliper assembly on the car. My only concern at this point would be if the correct and brake balance valve were installed. Dead give away on the correct master cylinder is that the front fluid well is noticeably larger than the rear one. If they both appear to be the same size, the master cylinder is intended for a drum brake car.