Maybe if you hot wired the engine to start you could eliminate some of the variables.
Please tell me if I'm wrong.

This might not work on a 122 if it has the armoured cable still in place. But generally a jumper is run from the battery positive to the + terminal of the coil-a temporary coil could be fitted to accomplish this and would eliminate more variables, then "short" the battery terminal of the starter to wire on the solenoid (when I was much, much younger this was accomplished with a screwdriver! (lots of sparks, melted threads and generally a low born but exciting alternative to a key). The engine should turn and start. There are much better ways of course. In my early Fords the solenoid was mounted on an inner fender and was equipped with a push button that could be used to turn the engine over from the engine compartment. Any length of wire with a switch in it would act the same.

If the engine still persists on spinning even after you disconnect the coil to battery wire then I'd say the problem lies in the starter. If not the switch or the wiring harness.