The headlight switch has nothing to do with ignition
The voltage drop you measured is way too much. One thing you can try is run a good heavy clip lead direct from "battery" (such as starter relay stud) to coil + and see if it fires. Then transfer the clip from the coil + to the "key" side of the ballast. You can easily find that, if the wiring is taped and hidden, by turning the key on, and measuring both sides of the ballast to ground. The side with the highest reading is the one you want. Clip your lead from there to the battery and see if she runs. If so, you have an ignition harness problem, IE bulkhead, ignition switch/ connector, or even the so called "in harness" welded splice
Another thing(s) to do if you have not is to remove, inspect and replace the bulkhead connectors, and remove all the ignition system connectors, ballast, ECU, and dist. and "work" them in out to scrub the terminals clean, and to "feel" for tightness.
And check the dist reluctor gap, although I know you said you tried another.
