Need some electrical help with my ‘68 Plymouth Valiant 100

You normally do not want the 60A "big" regulator for smaller alternators as the two draw differing field current. But nowadays, car parts are not what they were. If you buy a parts store replacement regulator, it will probably resemble the old one, but may have a shorter case, and will be solid state instead of electro mechanical. Re-read what I posted earlier about voltage drop. That is the major cause of over charging and pulsation. The old electro mechanical (relay type) VR can cause that but is not the prime cause, usually