If the engine won't run for a few seconds by pouring a little gasoline down the carburetor throat, you've got an ignition or valve timing problem. Just having a good spark isn't enough- - - -it has to happen at the right time, when the piston is at or very near the top of the compression stroke. Are you sure you have the spark plug firing order set correctly? I seem to remember the 503 I worked with having a distributor that rotated CCW, possibly, or maybe it had a non-standard firing order? Have someone tap the starter with the distributor cap removed while you watch for the direction of rotation of the rotor.

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!