And don't forget that grounds are also a "connection"- make sure all your grounds (esp. including the VR case) have a good solid ground to bare metal; no paint or corrosion in the connections.
And depending on how the vehicle is equipped, and with what parts; you could have mismatched components. Especially with a '66: for example, a later dual field alternator- usually all that you can get as replacements for the early single field units- are not correctly modified or the second field is not properly grounded.
A better description of the vehicle and any replacement parts and/or modifications already made is in order.