Keith,
All of the suggestion so far seem to be right on as the most likely cause of your problem. Some suggestions for isolating electrical problems.
1) grounds- check all grounds, make sure your engine is grounded, the regulator has a good ground etc. because of the design of these cars, the frame acts as the primary ground plane. A good heavy duty ground from the battery should be used to insure proper grounding......did I mention that the ground is important
2) Check all other electrical connections to insure that they are secure and clean
3) Check the voltage output of the alternator at the battery, with the engine off check the voltage at the battery it should read 12-12.5v nowwith the car running is should be approx. 13.5 - 14.5v.@2000rpm. Turn on the headlights and the voltage should drop and then recover.
4) If the voltage reading is greater than 15v the check the regulator ground, if it is ok the problem is Alt. or Reg. or wiring. If the Alt. is internally regulated replace the Atl. If it is externally regulated remove the connector and check the voltage at the abttery again with the engine running. If it dropps the regulator is bad. If the voltage remains high there may be a short in the wire between the Alt. and Reg. or there's a short in the rotor or stator in the Alt.Check the wire and if its OK change the Alt.
If the voltage is lower that 13v while running, a full field test can be performed which basically by-passes the regulatorand sends full voltage to the Alt. field (rotor). To perform this test (remote regulator only) disconnect the regulator connector and jumper between A and F terminals. If the output is normal during this test you know that the Alt. is good (bad Reg.) If the voltage is low the Alt. is bad.
(Test Information is from Haynes Techbook -Automotive electrical Manual)
note;If you are using an Optima or similar AGM (Acid Glass Matt) spiral design its important that the charge voltage does not exceed 14v and that you ONLY charge the battery at 1-2amps (trickle charge) over charging these batteries will cause them to fail.
Good luck hope some of this helps,
Mike "monster"