I am building a reverse trike van.
http://www.madabout-kitcars.com/foru...8&postcount=31
http://www.madabout-kitcars.com/foru...9&postcount=32
The model in the posts has changed as I now have a windscreen that is from a Leyland Lynx bus. The screen is almost 1m tall an so the shape of the cab front, in profile, changes quite a bit and the angle of the Land Rover door tops won't fit.
I am also thinking of it being more of a 'curvy' shape and so that requires the doors to have a more shapely profile. It would then be easier to not use glass that I can not cut to shape. Also give the wheel arch in the rear corner of the door I an unable to use wind down windows from a conventional car door.
I think you may be right about the 'where fitted' aspect as I could just build it without door tops and leave it open for a 'side screen' to be fitted later.