Why wouldn't you remove the starter motor and simply motor test it? (2-1/2% of all non-starting issues are electrical and due to mucked up wiring. I'd say start with testing the battery at a shop. While power disconnected verify the wiring is all correct. Is the system OEM 6V or did you switch out to 12V? Is the OEM Ballast Resistor in the circuit regardless if 6V or 12V - it is required for both. You can start with Essential Manuals. Parts, Service, and Owner Manuals are required tools. Once YOU know what you are dealing with, tell us more.
FORD 9N/2N ESSENTIAL OWNER/OPERATOR/PARTS/SERVICE MANUALS: