actually there are differences 39-40-41. the 39 cab has no drain holes in the windshield corners, the windshield regulator comes out in a different spot(horizontal instead of vertical) the 40 is similar to a 39 with the exception of the 2 previous things. the 41 is different at the cowl/firewall the firewall actually sticks out 1 1/2 past the cowl compared to a 39-40 firewall that is flat. if you are asking because you want to interchange the front ends you will have to do some modifications to make the hood latch stuff work.
