Stuck valve?--340 V8---Compression check RESULTS

The 60/50/60 number is low but the rest are within a reasonable range of each other. A simple test would be to squirt some oil in each cylinder and repeat the test. If they all come up then you know you have worn rings. If they do not change appreciably then I would say that the 60/50/60 cylinder has a valve leaking.

The rest of the cylinders being low could be a case of a cam with a lot of over lap bleeding off cylinder pressure. A leak down test will tell you if the low readings are do to the cam bleeding off pressure or if the engine is worn and its leaking past the rings or valves.

In anycase those readings would not be a cause for your back firing and rough running.

A timing chain jumping "a" tooth on the cam sprocket would be like retarding the cam 9 degrees, it would have no low end power but would run pretty good on the top end and it wouldn't account for your back firing. If the chain was to jump a tooth on the crank sproket that would be 18 degrees and you would likely not be able to start the engine and if you did get it running it would barely run.

I would start looking at fuel and ignition. As I mentioned before those wires look pretty suspect around the boots.