I've done that job and it's no fun, see here ...
http://www.gwim2.pwp.blueyonder.co.uk/td20.htm
Best solution is a donor cab section. That particular section is common to all 39-47 Dodge US, Canadian, military, and civilian hard cabs.
To do it 'right' you have to open the doors and drill out the gutter spot welds, do the same for the inner wiper panel, and then you can get into that joint and rework it. The roof seam is a triple panel. there is a back bit, a front bit, and the original seam is kind of a 'Z' shaped bit of tin which was spot welded to one side, and then the lot spot welded together. Have to say the originals I've taken apart were very approximate too.
The screen pillar sides are very solid (= thick ) but the roof isn't. If you had a donor cab you could split it at the top of the firewall and just fit the lot, which would save having to relocate the screen accurately.