Look at the ignition. When the starter is cranking, a terminal on the starter solenoid provides direct battery to the ignition box. In the ON position battery is fed through a resistor or harness-embedded resistance wire. If that feed is broken, you'll get spark only while cranking.
Could also be a marginal ignition box (what year? Dumpaspark or PrestNoLite?), marginal coil, but that seems less likely.
Duraspark boxes especially fail in strange ways. I keep a spare in my Rambler.
After all that cranking, it could be flooded and then a weak spark would not fire it; it didn't necessarily get any worse.