If you insist on keeping the outdated charging system I would suggest you check the continuity on the blue wire that goes to the alternator. It is the sensing wire that tells the regulator how much voltage is in the circuit and lets the regulator adjust the amount of charging time the alternator is on or off. If the continuity on the blue wire is lost the alternator will default to full output and overcharge.The regulator can also cause this condition as well as the red voltage wire from the main junction to the regulator.
The regulator is mounted in rubber plugs and must be grounded by a separate wire to the radiator support and also to the alternator itself. Sticky points inside the regulator can also cause the overcharge.
Converting to an internally regulated alternator will eliminate this problem as long as it provides enough output to meet the demands on your system.
Here's a thread on the conversion.
http://67-72chevytrucks.com/vboard/s...d.php?t=417872