At the risk of showing my ignorance because there are a lot (well there used to be) of people here who know much more than I, but here's a few questions.

I'm going to say by all the voltage checks you've done that you're way overthinking this. If you have spark at the plugs even if it turns over slow, it should eventually start. Another 6v jump might be nice to have about now. So assuming it DOES start, just doesn't fire right up, I would say you need to move on. So have you done some of the obvious things like looking at the plugs. Are they showing rich or lean or oily? Have you checked/adjusted the valves for proper clearances? If you haven't, be sure to really really bring the engine up to temperature. I recommend a nice lap or two around the neighborhood first. Have you checked and/or adjusted your timing via vacuum? I have had far better luck with a vacuum gauge than the timing marks. It doesn't usually happen but you might want to check the static voltage at the point with the engine off. Should be pretty close to battery voltage on one side and ground on the other. The ground side should show zero ohms to chassis ground. Both are assuming the points ARE NOT touching. If the gas is old, you may want to drain and replace. And if and when you do that, it might be a good time to inspect your tank. If you can get at the sending unit that's a real good peep hole. And if your gas gauge is bad, this would be a real good time to get it working properly. YouTube has some great videos on how to do that. And it may not need ethanol free but it might not hurt.