It sounds as if your pump has too much pressure. It may have failed in a too-powerful-supply way. It should send about 6 or 7 lbs of fuel pressure to the carb.. Any more and it can overcome the needle valve that the float closes.
I'd get a gauge on it, then you'll know. Don't just install the pump til you are certain.
I'm glad to hear that you are using the right switching to manage the pump operation.
Good going.
And to answer your question about when the pump runs, it runs constantly when the key is on and oil switch has pressure. On my engine if I have serviced my carb and I crank the engine for a few seconds, the oil pressure comes up. I can then pause while the pump runs til the oil pressure falls off. Better than long cranking in my opinion.
I have mine set up kind of hot rod 'ish. There's a marine pump wired and color coded like Mercruiser, it has a solenoid operated power supply. Then there's the fuel water separator, a fuel pressure regulator, a mechanical gauge and the carburetor. I went overboard with all the extra bits because I didn't trust that the pump had the right pressure. And I was right, it was high on the pressure. So now I can look and be sure everything is correct.
And yes, you will have to remove spark plugs to get that fuel out of the engine.
