It should be one of these two firing order. Try them both my 1969 351w base 408 stroker runs the later 302HO firing order. If it is 180° out it will spit sputter backfire and try to start. Go back to the basics; you need spark/fuel/compression and happening at the right time to run. Looks like you verified the timing is correct but the firing order can still be in question. You have spark at the spark plug so that is Ok. The spark plug wires correct on the dist? You said fuel at the carb so that should be ok…do you have it flooded? possibly fouled the plugs cranking without starting. Have you checked compression?
