Need Charging System Help


I built (still building) a 53 chevy truck. From scratch. I used the motor, steering, trans, wiring and pedals from a 98 mustang 3.8. Every thing works except the alternator isn't putting out more than 12V New alternator, new battery, new alternator wire, all fuses intact. Also grounded from motor to frame, frame to body, battery to body, harness to body. Any advice why the alternator isn't putting out enough juice to charge the battery. thanks in advance fellas.
Heck, first thing I'd try is to get the alternator and the battery tested. Wouldn't be the first time a new part was bad.