First, make sure it is not just firing while you are cranking the starter and dies when you let off. You won't hurt the starter by holding in on for 10 sec to test. If that is the case, your ballast resistor is broken. Common problem. It is the white ceramic part on the firewall near the brakes. Cheap and simple to replace.
If it runs for >5 seconds on the ignition and then sputters and dies, that indicates the spark is working and you have a fuel problem. Could be either too much or not enough. Will it run if you hold the throttle open, but die when you try to idle? When starting my 273 engine recently, it would idle ~30 sec, then die. I found the float tang was bent in the new carb so it was overflowing the bowl and making it go rich. If I had a helper, they probably would have seen black smoke out the tailpipe then. Yours could also be lean. Unscrew the inlet to the carb and check the rubber needle float (don't drop it) and fitting. There might be debris clogging the inlet limiting flow (lean), or keeping the needle from seating (rich) or the the rubber on the needle tip may be old (rich).
