You are right iceburgh. I am in the process of adapting a vent window from 51 to 48 door. First I made a template to make sure the window cavity on the 48 was the same size as the 51. The notch was easy. I just put a dolly under the lip of the door and used a hammer to bend it the same as the 51 door. Then I fabbed out the angle bracket with welded nuts underneath to accept bolts that hold down the vent same as the 51 door. No cutting required. The devider vent channel had to be trimmed a little at the bottom to match the angle of the inside brace between the inner and outer door skin. A simple flat brace was bolted to the
brace and the bottom of the channel. As czcheman said 51 door & mechanism fit the 48 door. I did a dry run the other day and so far it looks like things are lining up. When everything is rebuilt, painted and installed I will add pics.

1951 Canadian Chev 1/2-ton Shortbox