You can see if JC whitney has a replacement floor panel or
i just looked in my catalog and Mill has one, cab floor pan 39" X 25" but not for a 4X4
they also cary numerous other parts sheetmetal and some gas tanks plus senders for gm vehicles plus trucks.
I would buy the replacement panel, get a cutting wheel, cut the old floorpan off and weld in a new one. Or you can do what a guy paid me to do. Go to home depot, buy sheetmetal and use rivots and do a cheap temporary fix. I painted it and sealed it with silicon from the hardware store plus rubber undercoating.
If you have a mig welder already, i would buy a replacement panel, and spot weld it in, then later weld in the places that havn't so it doesn't warp. Seal the seams with silicon then paint it. Then undercoating the panel from underneath.
Things to look out for, brake lines, fuel lines, and anything that you might cut into or catch on fire. Disconnect your battery also while welding.