My '55 GMC won't start!! Help Diagnose

I have a 1955 GMC 450 Series with a straight 6, 302 Engine. It won't start.

After sitting for several years I rescued it and got it running after replacing the points, coil, cap and rotor and a battery of course. During the latter half of the winter it wouldn't start.

It attempts to get going and sometimes does for a second and stalls out. About every 5th try or so it backfires.

-The spark is excellent
-I think it's gettting enough gasoline

This is my first fixer upper and I am learning as I go. Does anyone have any suggestions or things to try?

Ignition timing?

The carb may be getting gas, but are the plugs? Could have some junk in the carb that was sucked in from the tank. Pretty common with older vehicles that sit for a while

I'm with macktoschool with this one. IF the wires are on the distributor cap in their proper position, AND the timing is very close to being correct, you probably have water in the gas tank. Go to the hardware store and buy about 6 feet of clear plastic tubing...the very small inside diameter stuff about the size of a pencil. It should cost about $1.50 or less off the bulk roll. Straighten it out using the heat of a hair drier or whatever. Once you get the tubing straight, stick it down the filler neck and into the gas tank. Gently suck on the tube until you see fluid. If what comes-up is rusty looking water or rust color sludge, then you'll have to drop the tank out for cleaning at the radiator shop or tank shop. Also, if that was the situation, the fuel line from the tank to the carburetor has to be cleaned too, and the carburetor has to be taken apart and cleaned. You can blow the fuel line out with carb cleaner tightly sealed to one end of the line. When you get it all back together, be sure to install a good fuel filter too. Sorry, but this situation is the probability. You might try putting about one tablespoon of gasoline down the throat of the carburetor. If it fires-up okay for a moment, that would confirm the gunked-up fuel system situation.