Check the continuity in the ignition switch. J1 is power in. There should be continuity to both the starter relay and the ignition 2 wire when key is in start.
Or if its all assembled, have someone turn the key and check for voltage at the matching terminals on the ballast resistor and starter relay with key in Start, then run.
'74 had the seatbelt interlock os the start relay wiring is a bit messier. There is/was a bypass switch if you haven't eliminted or modified it.
