best type alternator?

If what you have now is adequate for your needs rebuild current alternator or replace with similar. I've driven many miles with a 35A roundback and it's adequate for a stock 65 Dart, if only just.

The single field solid state VR is an improvement, far fewer voltage fluctuations and spikes (better with electronic ignition). The squareback with one field grounded is an upgrade over a roundback. A squareback with both fields running to a dual field VR is even better but requires some wiring.

A 60A squareback is plenty for a fan, more powerful lights and a reasonable stereo system. You can spend more on fancy high output alternators but they're overkill and bling in most cases.