I have been doing some unrelated auto electrical work on a 1964 pontaic bonniville .I checked the charging voltage with the giant 389 at normal idle .Measured 15.9 Vdc on trusted DVM .When we lifted engine revs to about 2700 and turned on the headlamps the voltage remained at this seemingly too high 15.9 Vdc.I have seen 15Vdc on older classic cars with generators .This detroit iron has an alternater with external volt reg which is normal for its year of manufacturer .The car is really origional and I want to keep it that way .I could rebuild the internals of the volt reg setting up at say 14.8Vdc keeping the car good for open bonnet concource .Is this voltage normal?
No, that is to high for that era.
13.7 to 14.4V maximum - newer cars have fancier control (I have one) and it charges up to 15.3V but the ecu will control to a set charging algorithm.
Those regulators will just control the voltage to a given point.
Some of those external regulators had an adjustment - even some alternators had 3 connections to give low, medium and high outputs - usually based on use: low for continuous output ie lots of highway and high for stop / start situations.