You can run a newer dual field square back alternator with your current single field voltage regulator setup. Hook the voltage regualtor wire to one of the two field terminals on the alternator and ground the other alternator terminal and you are good to go.

I believe the 75 or 80 amp Powermaster is too much for stock wiring. Even 60 amps will cook it if the bulkhead connector is not well cleaned and greased. You would be running up to 80 amps through 12 guage wire and a 3/8" spade connector at the bulkhead. With the goal of keeping it simple and cost low two suggestions.

One would be to get a reman 60 amp square back alternator from the parts store or other source and thoroughly clean and grease the bulkhead connector terminals. Also clean and tighten the ammeter connection studs.

The other suggestion is to get the 75/80 amp Powermaster unit and add another charge wire from the alternator stud to the fender relay. For this application, 10 guage wire with a short length of 14 guage fusible link added would be plenty. The added charge wire would effectivly bypass the ammeter and greatly reduce load on the bulkhead connector. With this modification the ammeter wouldn't work and you would need to add an aftermarket volt meter hooked to a key switched power source to monitor system condition.