1974 Slant 6 backfiring through carb.?

Was running ok when you shut it off last?
I'd look at the dist. cap for cracks and carbon tracking.
Wet cap, wires?
Maybe the choke is stuck open, resulting in a lean backfire when cold?
To put out a carb fire, crank the engine with the throttle closed.
Just a couple ideas. Keep us up to date.