apbmalibu,
I don't think the composition of the modern gas will make it leak any more or less than 'old' gas. A functioning fuel pump should fill a empty carburetor in a few seconds.
If the truck sits outside, my first thought would be moisture on/in the ignition system. Is your rig equipped with a choke, and do you use it?
Putting a electric fuel pump on the rig would be low on my list of remedies. If the engine is in good shape and has been tuned recently, it shouldn't take tons of cranking to fire it off.
With the 6 volt system, electrical integrity is pretty important. A long crank doesn't leave much "juice" left over for the ignition system. This is why everything must be in tip-top shape.
If you are bound and determined it is lack of fuel from sitting, and you have the energy, pull the air cleaner next time this happens and you will be able to see inside the carburetor.
If you work the throttle before cranking, and get no squirts from the accelerator pump, your theory about a leaky fuel bowl is true.
Crank the engine for 4 seconds for so, and do the drill again. The pump should have filled the bowl and the accelerator pump should squirt.
With the carburetor full and operating, you should be able to start the truck. Good luck.
Stuart
