Voltage rising then dropping? (67 Barracuda)

Check for voltage drop first in the harness and then the ground circuit. To do this

With key in "run" but the engine stopped. stab one probe of your meter directly into the top of the POS battery post. Stab the other into the IGN terminal of the voltage regulator, or if you can't get into that, the "key" side of the ignition ballast resistor. You are looking for a small reading, the lower the better. Anything over .3V or so (3/10 of one volt) you need to look for the cause. The bulkhead connector, the big red and big black ammeter wires, the "run" wire where it comes through the bulkhead, connections at the ammeter and the ammeter itself, and the ignition switch terminals and the switch itself.

Next run the engine at high idle to simulate "low to medium" cruise RPM. Do this test first with all acessories off, and again, with lights, heater, etc powered on

Stab one probe of your meter directly into the battery NEG post. Stab the other into the mounting flange of the VR. Stab hard, to get through paint, rust, chrome, etc

Once again, you are looking for a low reading, lower the better, and zero is perfect.

If you need or want to improve the ground from battery to body, look at the pass side front of the head. Those holes, unused, are also on the driver's side, REAR of the head. You can attach a Ford style "starter cable " (eyelet to eyelet) to the head, and bolt it to the firewall, maybe the master cylinder mount studs