The difference between the 39/40 frame and the 41/46 frame is the holes for the cab mounts. The cab is set back 1 1/2 inches on the 41/46 because of the extra bump out on the firewall. You will just have to drill the holes. I’m not sure about the total frame length. You will also need the front spring mounts to be completely correct, the springs and shackles are the same but the front mounts are different. The 41/46 nose extends out further and covers the bumper / shackle mount. They are just riveted in so it’s not a hard swap.
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