AWOL voltage regulator.

I used the same "square-back" alternator on my 1965. I wired it for the "2-field wire" mode ("low-side control"), using the 1971+ VR w/ triangle connector, all from junkyard. I put the VR on the inner fender right beside the alternator to keep things simple, and I was rewiring the whole engine bay.

You could also use the earlier VR which uses "1-field wire" mode ("high-side control"). You can buy electronic versions that are thin and shiny, or ones that look like the original big black box. See rockauto for all above, even the triangle connector. I don't know that the 1-wire VR has the oomph to drive the square-back's field, but "should work".

Whatever you do, lose the silly same-color wiring. The right wire probably goes to +12 V IGN, which is available at the ballast. It is currently wired for "full field", which will over-charge your battery (convert H2O to H2 & O2). I did that once to drive a car home when it wasn't charging, but "for emergency use only".