how much alternator do i need?

One big question will be "what do the fans" draw? That will have a big effect

The next thing is, "exactly what" is this 65A unit? Some of the older ones had poorer low RPM performance, and that's a big factor for many. "They don't like" dimming lights at low RPM. For many of us, "it was normal life" back in the day.

External / Internal regulator, assuming either system is wired correctly will have no effect. I do NOT however like "one wire" alternators as a general rule.

YOU MUST UPGRADE your wiring for any of this to be very meaningful. Please read the MAD article:

http://www.madelectrical.com/electricaltech/amp-gauges.shtml