Yes, I'd be checking for spark. You need fuel, compression, correct timing and spark to make the thing run. Sounds like you've got fuel and it also sounds likely that your timing and compression are okay, so check the spark. If the spark if weak then you have to look to the cause.
Hmmm...you know, it sounds like it might be a ballast resistor problem. If it's playing up all on its own, or if the coil is partly shorting, then you can get exactly the condition you're describing. Won't fire up, but wants to fire up just as you let go the key to stop the starter solenoid from sending juice to the starter. I had one that did exactly that and were it not for Dad telling me how to sort it out, it would've driven me nuts.
Anyways, you can check to see if the ballast resistor feels pretty warm after you've cranked for a short while. If it's getting too warm then that's likely the problem. The car's old enough to be running into annoying little problems like this. But in any case it's very likely electrical rather than mechanical, which is good news as it won't be too pricey to put it right. At worst it might be a matter of changing both the coil and ballast distributor.