[B. W.]

Let's keep it simple.
- Forget the body ground straps. They are not part of the starter circuit.
- Using a volt meter, measure voltage drop on both the power side & the ground side. ( red lead to the positive terminal, black lead to the positive lug on the starter.) Disable the engine so that it will not run (ignition coil) Crank the engine.

Do the same for the ground circuit. black lead to negative battery post, red lead to starter frame (ground) Crank the engine. (It really doesn't matter red or black, you can swap them, it will still read)

In "Perfect World" you will see ZERO volts on your meter. Any reading up to 0.5 volts is OK, any more & you have excessive resistance in the circuit. This can be caused by loose connections, dirty terminals, bad cables. ( I worked on an old Nova that dropped 8.0 volts on the positive cable)

Last, check voltage at the small wire from the ignition switch to the solenoid (purple) Red on batt positive, black on small small post on solenoid - should see close to battery voltage when cranking.

If the readings are good, your circuit is good. No need to change cables, swap parts etc. This test is simple & quick. With a helper you can test the entire circuit in under a minute.

If the electrical circuit is good, focus on the battery or the starter.

You can eliminate the battery by having a shop (NOT A PARTS HOUSE) perform a load test on it (preferably with an old carbon pile tester like a VAT 40)

Some shops have starter test benches also, most are long gone now. Nobody rebuilds them anymore. But, if all the above test good, you can assume it's the starter.