Why is the Gen light on?

[jayoldschool]

Taking care of the last few things on buddy's 63 C20. We bought it with a 350/350 combo, but we pulled and swapped in a fantastic low mile (30k!) 235/SM420 from the 62 C30 that we got in a package deal.

Looked like it had been converted to a later three wire internal alt. We converted everything back to stock following the FSM diagrams. Horn relay, regulator, correct ext reg alt, wiring. Gen light comes on when started. Ordered another regulator, no change.

Just looking for musings or experience that will point me in a new direction.