One theory is, if an exhaust valve is too loose or the retaining nut came loose and allowed the exhaust valve to loosen up then it may not be opening enough not allowing the gases to exhaust out causing the backfire when the intake valve opens which in turn will cause the fuel mist around the carb. What do you think?

I also think points can cause this to.

The truck was running fine for about 5 or 6 miles then all this started happening.

52 Chevy 3100 5 Window, LS Swap, 4spd Auto, Vintage Air AC / Heat, CPP Power Front Disc Brake Conversion with bearing upgrade, Power Steering Conversion, LED Tail Lights