Single Wire Alt setup to 65 Barracuda Harness

Monkey Madness - I did a similar set up a few years back and I took the alternator to my local Alternator and starter guy and explained what I wanted to happen; A newer alternator in an older vehicle. He changed the voltage regulator in the back of the newer alternator to a diff rent style that charged and regulated internally and there was only the larger B+ wire off the back. The other plug was filled with a dummy plug to keep dirt and debris out. Basically the engine reved and it "excited" the regulator and caused it to start to charge. It was a $50.00 repair and worked great on my plow truck for years. He was even able to "adjust" the regulator for a higher voltage for the plowing needs. I believed he adjusted it from 14.2vdc to 15.00vdc and in my old Dodge boy did it make a difference in the headlights. The alternator was a 120amp out put alternator I took form a new truck. After pulley changes and a few bracket mods.
Basically these internal were not parts I could buy locally. Made life a whole lot better for plowing and battery recharge. That might be a bit extreme for your application but the moral of the story is - where there is a will (and brains) there is a way. Let me add that the alternator only charge like 50amps at idle and the up and down of the engine speed on the older Chrysler alternators could not keep up with the back and forth of plowing and running the hydraulic pumps. The whole reason for the 15vdc was to stabilize the output a bit better at the lower engine speeds but did however kill bulbs faster due to running them at the end of their voltage limits.
Good luck,
Joe