Much as I would like to blame the Rochester B for everything from global warming to the national debt ☺ ☺, I really don't think the B is at fault here.
A backfire through the carb occurs when you have fire in a cylinder and an intake valve open.
Even though the wires were replaced a few thousand miles ago, I would look very hard at both wires and distributor cap. We once had a situation where the carbon in a set of modern spark resistant wires developed an airgap in one wire. The airgap acted as a capacitor, thus slightly delaying the spark to that plug. Would often run fine at cruise, but would backfire violently through the carb at light load (trying to maintain speed on an incline). A carbon track or light crack in the distributor cap could have the same result.
Let us know what you find.
Jon.
