OK, I'll take a stab at this...First off, I'm a 31 year veteran auto tech, so some of the stuff I'm gonna say may sound strange, and some may sound like a repetition of stuff you've already done. I'll try and keep it simple, here goes...
The only way that a backfire can occur through the carb is if there is a leak in the combustion chamber or a very lean air/fuel mixture. Lets assume the former for now. A leak of compression can happen one of many ways. I will list a few, but even though you said you checked these items, I'm gonna list them anyway. It is possible that you overlooked something, or misinterpreted your findings. Somehow ignited air/fuel mix is finding its way back up to the carb. The only way this can happen is if a valve is either open when it's not supposed to be, or not sealing properly. Thus you may have: A burned valve, excessively worn valve guides, improper valve timing, a broken valve spring, incorrect ignition (firing order or timing), wrong pushrods or lifters, etc...Therefore, we must confirm the proper sealing of the combustion chamber. The only way to do this properly, is to perform a "cylinder leak-down test". The tool can be purchased from Snap-On and other suppliers, and will tell you if you have a proper seal in the combustion chamber. Compression tests are OK for most basic diagnostic needs, but a cylinder leak-down test is much more accurate and informative. Even though you had somewhat acceptable compression readings (I like 140 to 150), there still could be a sealing problem. Some valve problems tend to diminish at higher RPM's (such as a small piece of carbon or a very slightly burned valve face) and others will get worse (such as a broken valve spring). The readings will be in percent. 10% loss across all 8 cylinders is about the max you want to see. More than that...you got a problem.
One other thing, contrary to popular belief, the engine could care less what towers the plug wires are inserted into. As long as the firing order is correct beginning with cylinder #1, and the piston gets spark when near TDC, #1 could be any tower! You see, it is possible that somehow the distributor was removed and installed such that the cap marking of "#1" no longer jives...So instead of fiddling with dist position, lets do this (much easier): Pull #1 plug (#1 is always the plug closest to the radiator, in this case the front plug on driver's side) and turn engine over with a breaker bar until you hear air rushing out (have an assistant hold their finger overthe hole) and slowly bring the piston to TDC. Next pull the dist cap off and note the rotor position. If needed, turn the dist slightly so the end of the rotor lines up with the nearest tower. This is now #1 regardless of what the dist cap says. Place number 1 wire in this tower and then follow along counter-clockwise in the following order: 18436572. 1357 on driver's side 2468 on passenger (from the front of course). Crank the engine, and if needed, move the dist SLIGHTLY one way or the other until it starts. Of course, all of this assumes that the cranking voltage is good, as is the coil, and you have adequate fuel delivery, etc, etc....

Good luck!