Sounds like a bad ground or wiring issue between alternator and battery.
Unless you are a purist, I would replace the alternatorand separate voltage regulator with a SI alternator from a 70's vehicle or even better a CS144. Then I would be sure you have a good ground from frame to engine block. With the newer, bigger alternator I would run a dedicated wire from the output terminal of the alternator to the battery. The bigger alternators do not work well with the factory ammeter, so you may have to bypass it. If you leave all the other factory wiring intact, you can leave it, but it may not be accurate.
http://67-72chevytrucks.com/vboard/s...d.php?t=425762
http://67-72chevytrucks.com/vboard/s...d.php?t=579118