Well the voltage test while the starter is not cranking is kind of meaningless. Voltage under solinoid load is the important thing. As soon as the load or current tries to flow the voltage probably drops off. The resistance in the original circuit is too high.

To end the trouble or suspected trouble. Wire a ford starter relay in the circuit to stop all the voltage drop across the thinner wires. Twelve volts to the relay and one wire to the solinoid. Common cure for your description of problems.

Everything has to be in really good shape for the original system to function properly. Your wiring and connections are old.

This starter relay is the one that for years ford bolted to the fender apron. It will see that good current and voltage arrive at the solinoid. The relay used to be about ten dollars.