If you can see the spark when cranking than the spark is fine. How would you know it wasn't sparking inside the engine? Unless you had X-ray eyes, a transperant head, or called Dion Warwick's Psychic friends, how would you know? I have a feeling it's fuel delivery. Do you have some "Quickstart" ether based fluid? Spray some quickstart into the intake and crank it over if it starts to catch it's sparking fine.
Like you said you almost got it running so either its flooded or not enough gas is coming through, did you prime the carbs? If not you're going to be cranking forever. I think you're mixing up the word "spark" to mean combustion. The plugs could be sparking merrilly away but if no fuel is in there you're SOL. Try the quickstart that will at least let you know that you're sparking fine. I'd also prime the carbs for at least 30 secs to make sure fuel is in the float bowls.
An alternative to quickstart is put a few drops of gas in the cylinder, just drops only don't pour gas in there, maybe 3 or 4 drops. Does it try to catch?