74 Dart won't start - ignition switch - maybe starter relay?

Not so sir. It was a pox upon my 75 as well. When I bought the car in 81 the first thing I did was bypass it.

Hmmmm, I’ve got a ‘75 and it doesn’t have the interlock. It’s a mid mid year build (Feb) so perhaps Chrysler ditched it by that point in the year.