Backfiring issue

I would say check the cam-timing FIRST, because if it has already jumped a tooth or three, the compression test will be LOW across the board; but without a history, you might not know if your numbers are low by design, or the chain has jumped.
In your case, you are not looking for a precise cam-timing measurement yet. You are just looking to verify a jumped chain. Nothing has to come apart yet, but your balancer TDC needs to be verified.

EDIT
Here is a clue;
Because the distributor is driven off the cam,
If the cam timing changes by itself, then
so will the ignition timing.
So if you come out one day and because the engine won't idle and runs like crap, you crank on the speed screw and then check the timing only to find that it is very retarded, THIS DID NOT HAPPEN BY ITSELF! Well it did, but the timing changed because the cam timing changed. So then, IMMEDIATELY a redflag should pop up in your brain. But instead, most guys will just put the timing back and wonder.
Of course, putting the timing back don't make it run much better, cause the valves are all out of time, and the cylinder pressure went on vacation..

I have personally had this experience, back when I was a young man, in the early/mid 70s .
I was called in to help with a 318 that would only barely idle with the 2bbl WFO, and the ignition very advanced. But in fact, changing the timing while the engine was ticking over, had very little effect, except the back-firing stopped. It didn't take long to figure it out, and when the front cover came off, there were just stumps remaining on the cam-gear, as the nylon was mostly gone. As I recall, the timing chain had jumped several teeth.
The point is this; it only takes a jump of one or two teeth to create chaos. If I could remember how many teeth are on that gear, then I could tell you how many degrees each tooth is, but, I cannot.