Backfiring issue

Just my &.02. Do some basic diagnostic checks, for example don't pull timing cover to check chain slack, pull distributor cap and put a wrench on the crank bolt, then assess how much you have to move the crank before the distributor rotor moves. This will lead to whether you need to go after the timing chain . IMO, its alot of work pulling that timing cover if you don't have to. Check timing, get a vacuum gauge etc, pull a valve cover and start it, see if all the rockers are moving smoothly and evenly etc. Don't get too invasive as far as tearing the engine apart not until you know you have to. Check and double check firing order, make sure wires are good. I've seen 1 year old Belden wires that look good but are shot. I use an inductive lead timing light to check and isolate the misfire, you just switch the timing light lead from wire to wire looking for the erratic strobe light flashes.
As you go through these checks, take notes at some point something will jump out and you will be able to isolate is it fuel or is it ignition, is it timing or is it broken or worn mechanical parts. OP you mentioned ignition, make sure you have a good ground> I run MP electronic ignition,and I actually made and ran a ground wire from my orange box bolt hole to the rear head bolt hole just to make sure there was a good ground to the ignition box. One other thought you mentioned back fire out the carb, is that constant like its all 8 cylinders or is it once in a cycle like it is just one cylinder?