Car backfires only on one spark plug?

So my ford 390 has a backfire (but only on the drivers side tailpipe…). Except for when I remove spark plug number 6. Doing so makes the backfiring stop. Also, I notice that the exhaust on that side of the engine is cooler and not as loud as the exhaust pipe on the other side. The car idles fine, but when you put it in drive it wants to stall immediately. Reverse makes it idle rough and want to stall as well, but not as bad as putting it in drive. I was planning on rebuilding the car (edlebrock 4 barrel) but am now wondering if the real issue is the misfire, and not so much the carb. What should I do first to try to remedy this? Any thoughts?

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