HELP: Headlight switch triggering STARTER!!!

If it was a loose ground, then for some reason it had current and was back feeding through something to the starter relay.
Off hand the only thing I can think of that the headlights would feed that go to an dash panel ground would be the instrument lights and the hi beam indicator.
I'm not really familiar with the seatbelt interlock box but it does bring a bunch of wires together.
Some of the other guys know it better.
First booklet here has some info about it
MyMopar - Mopar Forums & Information - Browse MTSC by Model Year - 1974

and unfortunately keep in mind there is a possibility some wire colors on your car may be non standard :(
See the TSB here The 1970 Hamtramck Registry "1974 DODGE MODEL TSBs Index" Page