my autoshop teacher in high school was teaching us about torque by brakestanding on a 454 equiped 1ton truck and instead fire came out of the carb..nothing happened to it as far as i knew, just..was an interesting show .
In my experience a backfire through the carb usually means you're running really lean. This can be caused by the carb being out of adjustment or from a vacuum leak.
If that's not the problem, start looking for an ignition problem (like stray sparks from a bad cap and/or wires).
I had a real problem with fire coming out of my carb. When there was even a slight load on the engine it would just keep shooting out. I ended up adjusting/readjusting the timing, changed plugs, wires, distributor, distributor cap, rotor, and even ended up getting a new carb. Problem still wasn't solved, it ended up being a head problem, most likely the valve seats.
I drove the car like a granny for a while 'til I could finally afford to get some new heads. Went w/ the Edelbrock Performer RPMs, a new camshaft and headers to compliment them and the car really screams now.