74 model has a one year only seat belt interlock feature also. Majority of those have been bypassed today but how/where it was bypassed may be a factor. That is the first place I would look at 74 model.
For the starter relay to close and run the starter, it needs 12 volts on a yellow wire when ign' switch is in start position ( interlock would interrupt this 12 volt path ) and a ground path via the brown wire through the neutral safety switch when the shifter is in park or neutral. A simple test light ( and maybe a helper at the switch ) should be all you need to diagnose.
