I had problems with my stock charging system. External voltage regulator would hang up and not allow charge. A tap with a screw driver handle would get it charging again. I swapped in a solid state external regulator from Autozone. I think it spec’d for a late ‘60s chevelle. That solved to problem and was a cheap easy fix. (Could be an easy test for you.)
But, I still had a poorly performing alternator. The original 10SI doesn’t perform well at idle. Although it charged well while cruising, at a stop light with the headlights on and heater fan going voltage would drop off to 11.5v and keep going down. So, I swapped in a 12SI per this diagram as outlined in the thread VetteVet referenced:
http://67-72chevytrucks.com/vboard/a...7&d=1360950747
Also look at this website, lots of good info:
http://www.madelectrical.com
I’m really glad I made the switch. The12SI performs much better. Voltage never drops below 13.5v and normally sits right at 14.2V. Also, using that wiring schematic I was able to maintain the stock ammeter.