I did just that on my '51. The method was dictated by the fact that my 5-window donor cab was almost a total rust-out, except the area around the corner windows. I am talking major swiss cheese here, on the roof and around beltline. It may well be easier to chop out the entire roof, but that was not an option for me. The plan I followed was to cut the windows out of the deluxe cab, then trim the outer sheet metal down to the minimum required for a good fit in the 3 window cab. The inner sheet metal was cut on existing seams only (where possible). It was actually more like removal of spot welds. So you have the donor windows out to use for a pattern before ever touching the 3-window cab. Carefully cut the 3-window cab and use the new pieces as 'plugs'. If you do this carefully, using the existing cab seams (they are in the same place on both cabs) as reference points, then you will get perfect alignment. I used a combination of a sawzall and cutoff wheel, a plasma cutter would be nice but is not required.
The new corner window sheet metal is then butt-welded to the old. Bodywork should be minimal and mostly in the roof area where there is a slight compund curve. Grind off excess weld and use body filler to finish it off. If done right you wont need much.
A bonus of doing it this way is that with the inner sheet metal removed, you have access to the pillar post and can easily weld in a reinforcement tab for shoulder belts. I will post some pictures of how it turned out if anyone is interested.
