1971 mustang pulls left when braking?
Sounds like you have a seized brake piston. Its the part on the brake that pushes on the pad when you add pressure to the brakes. Its probably not the rear brakes causing the problem because of the steeringwheel moving when you stomp on the brakes.
There are seals on the piston which can be replaced, or moisture has creeped behind the rubber protecting the piston and corroded, take your front brakes off and have them reconditioned.
Also, how tight did you do the bearing up ? If you jack the car up on the front, do both wheels spin freely ?