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.
