I can ONLY react to what POSSIBLY might be causing the misfiring and when you get to the point that ALL ELSE has been eliminated, then I will suggest again that the camshaft is the issue, being ground with the wrong firing order like a 1960's 289-302 firing order.
You would have to have degreed EVERY cylinder in order to discover that. Normally no one does that, just degrees in #1.
More and more it indicates TO ME that the camshaft itself was ground with the wrong firing order AND THAT THE IGNITION IS FIRING CORRECTLY.
IF THAT IS SO, you are not getting "backfires", you are hearing the spark plugs fire while the valves are still open. That is why some have suggested that the distributor is installed 180° out of phase. It certainly CAN be the answer.
I have a "friend" who is now retired but once sold aftermarket Ford racing parts and complete engines.
He once sold a 427 Ford engine to someone who had the distributor installed "backwards" or more correctly described as 180° out of phase. He had to accept the engine in return because he was accused of selling a "defective" engine.
The fact of the matter is that SOME engines are capable of running that way , on 4 cylinders, while most are not. I do not know if a 351C is or not. I never had to find that out.
I DO NOT like that answer either BUT just as Shelock Holes once said, "when all other possibilities are eliminated, what remains must be the answer".
You have already done so many things I can't keep up with what has been done and what has not.
You can actually test the theory that of the distributor being out of phase by 180° by changing the rotor in the distributor.
Ford rotors are made in two ways. One with the locking tab on the rotor pointer side, the other opposite the pointer. Whichever rotor you have now, find one with the opposite orientation, install it and see if the engine fires correctly then.
It can not hurt at this point.
Since it appears to me that I am not contributing to this discussion positively at this point, I will now retire from commenting in it at all and just wish and hope for the best.