Dave,
It is the original voltage regulator that came with the car, so I should be able to adjust it. I think I have a Nissan OEM spare that I bought back in the '70s but now I have to find it!
FYI, I just went out and measured the voltage at various engine speeds with a VOM that is probably more accurate than the gauge in the car and it seems that the voltage peaks out at 15.3V at 2500 rpm and above, I had 13.7V at idle (all with no load). At this point I think it is probably OK as long as I have some load on the system such as the fan or lights as that brings the voltage down to what I would think is a normal value.
Thanks,
John