You sorta' have two sections of the circuit, and you can divide the problem in half immediately
The first section is the key, and circuit feeding the key (ignition) out the bulkhead, to the starter relay (on the firewall) and down through the neutral safety switch to ground
The OTHER half of the circuit is from the battery feed, through the contacts of the (firewall) starter relay, and down to the starter relay.
BUT but but..... then last, you also have the main big battery cable to the starter "big" post
So divide the problem in half. Will it crank (turn over) if you short across the only two bare terminals on the relay?
If not, you have a problem in the big main cable to the starter, or in the starter/ solenoid itself.