I changed my 53 3100 from 3 window to 5 window. I used the entire roof and cab back, spliced in @ the rear floor/rocker and windshield. As stated earlier, the floor/hinge pillar is the starting point. I essentually built my cab around the doors, and windsheild opening. I focused on even gaps around the doors. So I would suggest you check the condition of the floor/rocker and hinge pillar and go from there. Hope this helps.
