Its an ork kit, as such as long as it looks like a plane, it can be whatever kit you want it to be.
I'd just build the plane the way you want to. Only don't glue the rear gunner hatch closed and make up one of the grot gunners for the burna/blitz bomber. Now just magnetize a few of the bombs for the wings (magnet on top of the bomb, another magnet embedded in the wing). The guns can just be magnetized by a magnet in the gun port and on the end of the gun.
As for where the tail fin, extra engine bit, wing pitch extentions, exhaust piping, it shouldn't matter which ones you use for any of the three versions. All the matters is the armament.
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