Although usually the intake valves and backfire into intake. A can of Seafoam in the fuel and fresh gas fixed those.
Also had one more like yours with exhaust backfire. Started out intermittent, gradually changed to constant. Plugs looked good. Wires, cap, rotor OK.
Flat cam lobe.......
Checking the lash will identify that quickly.
When you isolate cylinders by unplugging one plug wire at a time, if the miss doesn't change much (might get
a little rougher) but
the backfire goes away, you've found the cylinder. I'd suggest trying that first - at least you will know which valve cover to take off.
Good luck.
