Sounds like you may need to check the grounds on the regulator and alternator. Make sure the regulator housing is making good, clean contact to the firewall METAL. You may need the scrape the paint off the mounting holes to get this. Star washers help here.

Do you have the firewall to engine ground strap intalled? With the engine running, measure the voltage between the regulator housing and the alternator case. Since both should be grounded, there should be a negligable voltage drop bewteen the two. If there is voltage between the two, I would check re-check grounds!

Hwck if you have the tools, use a 16 ga or lower jumper wire with alligator clips to connect the VR housing to the neg battery terminal. If this helps the problem, grounding is the problem...try the same with the alternator case..

Grounds will get you every time with these cars!