I put a 140 amp 12si on mine and it works well, runs the fans, heater, lights etc. Don't have a boom box car so no amp though.
Just disconnect wires from the regulator terminals 3 and four and splice them to alternator terminal wires two and one then connect the output cable. Disconnect and tape off the #1 & 2 regulator wires.
# 1 is the alternator on/off terminal and #2 is the voltage sensing terminal. # one goes through the idiot light in the dash or a resistor wire if you have gauges. If you experience run on after you turn the ignition switch off put a diode in the # 1 wire.