Assuming the new plugs are properly gapped and new wires are good, and backfire wasn't there prior to installing the new parts, I'd be suspicious of the wires' routing.
In your firing sequence, 8 or 7 cylinder wires could be affecting 4 or 2, if they are running parallel to each other. Usually, they should cross. Try pulling these wires apart with the engine idling and see if this makes any difference. Also, try swapping wires and see if this changes the cylinder backfiring.