I think by most accounts people put a greater value on the 5 window cab. I know it makes a difference to me and most of the restored trucks of this era that I see are the 5 window. I also know that I have seen some article out there about cutting the cab at the belt line and the roof to fab in the back half of a 5 window cab to a 3 window cab. I also know that the vendors now sell a 5 window patch panel that would do just this. If you were paying someone to do this I do not know if the increase in value would offset the cost.
my .02
steve