Are you sure the firing order is correct? On later model Chevy 6 cylinders (216, 235, etc.) the firing order is 1-5-3-6-2-4 with the distributor turning in a clockwise direction. Also be sure the distributor rotor is pointing toward the plug wire for the cylinder that's on the compression stroke when the coil fires.
Have you checked all the cylinders for compression? A burned or sticky intake valve will let the fire from the cylinder back up into the intake manifold, and a too-tight valve adjustment on one or more intakes will cause the same problem. Use a bright light and inspect the inside of the distributor cap for "carbon tracks"- - - - -they look like fine pencil lines from one connector to another that can let the spark jump to a plug wire that's not supposed to be firing at the time. Good luck! Use starting fluid sparingly, if at all. A teaspoonful of gasoline down the carburetor throat will accomplish the same thing with a lot less drama. Good luck!
Jerry
