1971 Alt issues

Took the engine out of the car for a rebuild, when we put the engine back in the alt was froze up, no big deal, called the auto parts store and got a replacement. The old alt had 2 fields and the new alt has one field and a ground. I connected the new alt up, ground and all, now I'm having overcharging issues with the car, it maxes out at 18 volts.

My question is should I find another alt that has two field terminals or does this even matter? Do I need to look into grounding issues? I've read a few threads on this matter and there are a lot of solutions, but I just wanted to know, if I even got the right alt for it.

Thanks,