I would imagine the distributor drive gear is in 'wrong'. It's difficult to get it in the proper orientation, even assuming you know what the proper orientation should be (there's a picture of what it looks like in my Haynes manual - some odd degree with #1 at TDC). When you put the gear in it twists as it descends. And to complicate further it hardly ever engages the oil pump drive and goes all the way in the first try - you have to turn the engine around a bit until the pump engages and it drops that last 1/4 inch. All in all it's a fairly tedious and unrewarding task.

But in any case you can always move the sparkplug wires around and twist the distributor to get everything working properly regardless of the position of the timing gear. When the crank pulley indicates 'TDC' it means both #1 and #4 are at TDC - you'd have to check which cylinder has both valves closed (peek in the valve cover or take it off) to see which one should be getting the spark at that particular TDC.

Backfiring can also be caused by maladjusted carbs running too lean.