I know they are now making complete new cabs (for ungodly amounts of money). I believe they also make just the rear half of the cab along with all of the other parts.
I would just find another 5 window cab and rebuild that. You'll still most likely have to go through and fix the same things like floor boards and cab corners. Look for the old grain trucks too. Most of them were probably 3 windows, but, you might get lucky and find a low mileage truck.
