Are you building a 41-46 with the 39 cab? If that is the case the 39 cab will bolt on. The bolt holes in the cab are the same 39 to 46. As Steppenwood said the fire wall is bumped out 1 1/2", the indent in the center of the fire wall for the valve cover is the same 39 to 46. The cab on the 41-46 is mounted 1 1/2 inches back for that bumm out. If you use the 39 cab for a 41-46 with a v8 you will have the extra 1 1/2" in the engine compartment. If you are going with a 39-40 nose you will have to move the cab forward 1 1/2" and change the front spring mounts. Then you have the frame length and bed differances as mentioned.