71 Dart overcharging

My wife's 71 SB Dart overcharges.
All the usual issues have been checked.
Checked with meter so it is real. 15.5 volts at battery.
Doesn't come down after driving it
Replaced Alternator
Replaced 3 different regulators, all new
Ran temporary power and Field wires from alternator to regulator, to bypass factory wiring.
Correct ballast resistor in place.
Only thing not stock is, the battery is in the trunk
Good grounds
Good heavy battery cable direct to relay.
I also have a heavy wire from when we raced it, that goes direct to the battery from the alternator, eliminating factory wiring. I can run it on either wires, factory or direct
Still 15.5 at the battery
Any suggestions?