I simply use vise grips to clamp it into place with the firewall first. It took some effort, but I didn't have to reform any of the flanges. This assures that the pan is all of the way forward where it is supposed to be. I then screwed the pan down to the frame using self tapping sheetmetal screws working my way back as I went. I also did the flange to the rocker simultaneously. There will be some inconsistancy on the rockers, but when you weld and can easily shape the hot metal with a dolly hammer as you go. I also had measurements for the floor tunnel height off the original floor in the back so I could jack it up to the correct height for welding in the footwell pans to assure proper tunnel alignment. Didn't want my drive shaft to be hitting the floor tunnel. Looking now, the drive shaft really isn't close anyway. The footwell does sit on top of the front floor pan. I copied the original weld pattern off my original floor as close as I could. I pre-drilled plug weld holes prior to installing or clamping and screwing it into place. Overall, the fit was very nice and most people that see it can't tell the difference from original.
I hope this helps your out Per...