You will probably get a lot of opinions on this, but I would recommend using 16 Gauge for Floorpans-most of the Street Rod replacement Floorpans are made of 16 gauge. I would use cold rolled (easier to Paint and finish-if this were a Drag Car I would use Aluminum, but not on the street-gets dented too easily, rocks, debris, etc.)-
It would be better to have some sort of Flange to weld to, although it is not imperative-please Post some pictures so we can see what you are working with. I would personally let the Body be removable, but you don't have to. I would recommend you purchase a Bead Roller from Harbor Freight (to keep the cost down), look on here as to how to reinforce it (so it won't flex all over the place) and go to town.
As far as cutting the 16 Gauge, I would recommend a Kett type of hand shear-like this style:
They can be purchased cheaper, but you get the idea-the only problem with a Kett is that it doesn't like tight corners, so I use a Nibbler for that-you will get a lot of opinions on here about which Tools to use to cut Metal. I would Weld to the Rocker Flange if possible, and then make your Channel pieces from there. Good luck, it is a fun project!