Slant 6 confusion

Firing out the exhaust could just be flooded.

I would pull the plugs, disable the ignition system, and crank it until the gas stops flying out of the cylinders .
Then put a couple a shots of oil in each cylinder, let it sit for 20 minutes, while you dry and regap the plugs.
Then put the plugs back in, and fire it up. If it happens again; fix the carb.

Caution; if gas does come flying out, remember that gas likes to burn, especially when it is mixed with air! Don't set your truck on fire; disable the ignition.
If your muffler is full of gas, anything could happen.


Other causes not yet mentioned for "after-firing", are valve timing, rotor phasing/ distributor out of time, and carbon tracks/or just plain moisture in the cap. One thing for sure is that it does in fact have spark!