The ol' man has finally went off the deep end!!! Cutting holes in his new floorboards, what on God's green earth was he thinking. Maybe he built to many car models with the windows closed, who knows.....
Any way, putting in the cab support box is a real challenge, not impossible, but probably the most difficult of the project. Tied to the z brace first with zip screws, makes it easier to manage. I like to drill a 1/8" hole first so the threads don't catch on the outside layer of sheetmetal and walk up, thus separating your joint. After that is attached, you can pull back on the cab support to line it up with the leading edge of the floor board. Don't forget you have a doubler plate that mounts the front fender in. the doubler has to sit against the forward edge of the cab support, not the flange, but the leading edge of the cab support.
Don't be afraid to use a body hammer to tighten up the flanges and corners after it is zip screwed, they bend very nicely with a little bit of effort...
OK here's the pics.