I compared a 40- 1 ½ t. LWB Frame and a 41- ½ t. short bed frame and this is what I found. The front spring and bumper brackets that attach to the frame rails are different. However you can interchange them. American Classic list the same front springs for the 39 to 46. The front cross member is the same and in the same location. So the radiator support and motor mount would interchange. The holes for the steering box are the same in the same location and that will interchange. The 8 bolt holes 4 each side for the cab mounts and they are 1 ½” further back on the 41 frame than the 40 you would have to drill these as the location of the cab was moved. The frame on the 1 ½ T narrows between the third and fourth cab mounts after measuring a number of times I went and looked at the cabs. There are four rear mount holes (2 each side) for rear mount behind the gas tank in the cab on both the 40 and 41 cabs, which allows the interchange of the cabs onto the different frames.
I also measured the concave on the firewall that makes room for the valve cover. It was the same depth on both the 40 and 41 thus the reason for moving the cab to off set the firewall bump out. Also when I measured this I noticed that the 40 firewall had 2 vertical strengthening ribs on each side where the 41 had the ribs in a cross or X pattern.
I measured the 41 frame along the outside rail it is 167 ¾ “ I do not have a 40- ½ T short bed frame to compare.