These unregulated charging systems depend on load to keep the voltage down.
They usually put out about 9a at WOT. You can get a 50a bridge for a few bucks,
I will send you one if you want. I have a drawer full.
The 3 terminal regulator might work but be sure it can handle the unregulated
voltage from the alternator. If you could find a 1960-7x? Triumph motorcycle
zener it would solve the problem. I had the same thing with my old 75
merc/mariner. It used to bake batteries.