Before you replace more parts I would recommend you do some more troubleshooting. Backfiring through the carb can be timing OR carb related. I'd put a vacuum gage on the intake manifold and adjust the carb to get the highest vacuum (15-20). It will also tell you if the ignition timing is off. Set your timing at 12 deg., dwell at 30. Pontiac engines seem to like 12 over the recommended 9. Once you know your carb is tuned, the timing is close, make sure your plugs, wires, ignition cab and rotor is good.