dropped in a Power Master 140amp on my 70 Charger, 440 Non-AC, never modified a bracket for it. You gotta make sure its tightened good though. When I put my foot down the belt would tighten up, flex the alternator and the back of it would tap the cylinder head, kill the fusible link.

Clean up your engine bay, remove your voltage regulator. I run a Delco on my 69. Cleaner engine bay, less wires, Start the car, rod check to 1500rpm's. Alternator on and charging at 14.0v. 100amp alternator