The doors will interchange from 37-40, but there are a few peculiarities in a 40. Only the 40 has the vent window, or the outer door handle which fits in an oval hole. A 37 won't have any trim holes, but those are easily drilled.
My '40 has a '39 door on one side and a 40 door on the other... both with the 40 vent windows, so they can be interchanged.