[LongBox]

A backfire strong enough to blow a muffler means unburnt gas was in the exhaust. this is because the engine was turning, but not firing. When it started firing again, the hot exhaust ignited the unburnt gas in the muffler. We used to do that on purpose when we was kids.

Since the last thing worked on seems to have been the ignition, time to go over the distributor very carefully, in an organized way. Is the wire from the coil connected correctly? Is the rotor seated on the plate on the top of the shaft. Is the centre contact in the cap there, and in good order. Is the centre contact on the rotor in good shape. I know these are new, but they can get out of whack.

Next, is each plug wire firmly seated in the cap? And at the plugs? Are the wires in good shape, with no issues with the insulation? Etc.Etc. Etc.