cant get it started!
Your ignition switch should send 12 volts on the brown wire at the ballast resistor and the yellow wire on the start relay when in the start position.
If you are using your key to turn it over we know that yellow wire is good.
Both wires are routed through the bulkhead connector along with a crowd of other wires. One of the 3 connectors there is forward lamp harness another is wipers along with the forementioned yellow wire to start relay, reverse lamps, and neutral safety.
This leaves only 1 connector full of wires to focus on.
I'm not as familiar with the slant 6 wiring so if you had a loose wire at the starter or something like that, I wouldn't know it.
It couldn't hurt to double check stuff like that.
The 73 service manual will be the ultimate tool if you plan to do your own diagnose / repairs