To answer your questions about how to tackle a cab that has no bottom:
Reinstall the doors, or set them in place and get a uniform distance around the door margins and then tack some supports to the solid sheetmetal to hold things in position and remove the doors again. The Factory Assembly Manual also has critical dimensions that you can use as a reference. They didn't measure things with a micrometer at the factory, so there's some tolerance in the measurements. You can buy complete floor pans, which may be the way you should proceed if you're going to use the cab you have. That alone would save a bunch of time.
But I agree with Ron's suggestion of buying a replacement cab (available new, for a bunch of money), or a donor truck.
My truck is not quite as bad as yours sounds, but I took it on as a way to give me something to do in retirement and keep me off the streets.
I keep finding more patch panels that I need to buy after digging into it, so it's accomplishing the mission of keeping me occupied.
