alternator for 65 dodge dart

It really doesn't matter, kinda

Alternators do not need to be "matched" to the regulator the way old generators did, but

A great big alternator may strain the bulkhead connector/ ammeter connections. The big problem with parts store rebuilds are that you don't really know WHAT you get.

If you get a "rebuilt" I'd get a post 70, and ground the extra field connector. That way if you decide to upgrade to a '70 regulator later on, you already have half the battle.