If you are running a points distributor, by bypassing the resistor wire in the harness you are providing too much voltage to the coil and points. That shouldn't make it backfire, though it will prematurely wear the points. Also check or replace the points capacitor when you are done playing, along with the one on the coil and the one on the blower motor. One of those may have popped. Perhaps loading the intake and cylinders with that charge of fuel before turning the ignition on made the big blast.

I drove a car for two weeks like that, pushbutton and toggle. Lights are overrated, anyhow...