HELP: Headlight switch triggering STARTER!!!

Offhand I'd say "you are crazy this cannot happen" LOLOL

First thing I'd do is try disconnecting ignition switch at the column connector and see if problem goes away

Have you done ANY other wiring repairs?

One "source of entertainment" is 74 should have the much hated "seatbelt interlock." Look for a big box under the dash with a long inline connector and pull it loos, see if that changes the problem. On a side note, the interlock has a reset box under the hood, about the size of a horn relay. RED reset button. You need to permanently splice the two yellow-ish wires together which are the "start" wire going to the start relay

Has there been any wiring malfeasance such as smoke and smell? Burned wiring? damaged harnesses?