Engine missing on one

[75sierraclassic]

I really need your guys help with a diagnosis. It all started with a couple plug wires that shorted out on the headers. I ran a while with the bad wires (kept wire from touching header) due to extreme weather and nowhere to change them. It ran a little rough since it was missing on a few cylinders. I recently went through and replaced wires and plugs. Still missing. Replaced cap, rotor, and coil. Still missing. You can hear it missing at idle and it is real bad driving at low rpm in low gear at a high load (at 35-40 mph in overdrive). It makes a metallic ticking noise with each revolution that gets really loud at high rpm. The tick is obviously coming from the left side of the engine. Can't decide if that noise is internal or an exhaust leak. I did the tube stethoscope trick and couldn't hear the tick in the valve cover. It was also louder underneath the truck (leading me to the exhaust leak). I haven't pulled the plugs back out to see which cylinder is missing, but with new plugs wires cap and coil I'm starting to think its a lifter or valve or something. It doesn't feel like the cylinder is totally dead though, it misses sporadically. Sorry for the long post, but if you guys could give me any input I'd greatly appreciate it!