On your next cold start, remove the air cleaner and hit the carb with a shot of starting fluid. If it starts immediately that tells you that you're losing fuel somewhere, probably draining back out of the carb, or boiling out after shutdown as carbking suggested. A transparent fuel filter near the carb also will show you if there's any fuel in the line when you crank.
BTW, you don't by any chance have a Pertronix points-replacement ignition in that, do you? Their older models used to have that idiosyncrasy.
I have done this for a long time after my truck has sat for more than about 2-3 weeks. Remove the air cleaner and spray a mist over the top of the carb for 1-2 seconds, and it always fires immediately. I figure this is much easier than hard cranking on the starter.
Also, a few years ago, I was told by a starter/generator/alternator rebuilder to never crank for more than about 3 seconds at a time. Cranking for extended periods (8-10 seconds) burns up the internal wires and connections. Don't ask me how I know...
