It can be done as I have done it but it is not as easy as it looks. The 38/39 cab is narrower so the 40 hood is too wide at the cowl by about 1" I pinched the hood at the rear to make it fit.
You need the 40 running boards and radiator too, one hole on the fenders lines up with a hole on the A post, the rest have to be drilled as the curvature of the 40 fender comes up higher. The 40 inner fenders fit the frame as the frames are the same So if you go for it, get the running boards and inner fenders mounted up first as these are your reference points for everything else. One of the biggest jobs was making the hood hinges work.
After I did it I kind of regretted it as it was so much work, I think that a swap of an earlier car front sheetmetal say 37 or 38 would be easier and might be more of a bolt on job. I have seen pictures of both of these swaps on to 38 cabs.