Has anyone here replaced floor pans in a 64 or 65? Looks like a huge job. Is it doable without removing the body from the frame?

I've been searching around , with no luck finding a replacement roof panel for my 78 el camino . I have only found weld in replacements for 68-72 . My original idea was to cut out the rusted panel , and have a sheet metal replacement formed and welded in its place. I need some direction ...