Iv'e been kicking the big window upgrade idea around for my 59 F100 also. If I do this, I would do the whole roof and entire back of cab so the only welding would be at the top of the front window pillars and the back of the floor pan, under the fuel tank. Does this sound reasonable? My floor pan and firewall are in good shape with no major holes, but my roof is rotted above the front window and the back of the cab has holes in the bottom corners so that's the basis of my idea.