[Mister T]

Reading thru the posts I do not see that you looked at the distributor for a worn points mounting plate or loose shaft as Malb suggested , as bad as this engine backfires it as to be an ignition problem , possibly a rotor arm that is shorting to the shaft , I have seen this many times with cheap ignition parts .
If you have a dwell meter check to see if the dwell angle changes radically , approx 5 degrees max should be Ok , but do look at the rotor for tracking where it fits to the shaft if in doubt instal a new Bosch part.
One more point to check , that the coil is getting 12 volts constantly especially when the engine is backfiring.
Mister T.