Brian:
That's a lot of rust...and you've already got some patches...make sure the tub is solid before you go bolting-in a larger cage...otherwise in a roll-over you'll just break through the floor.
From what I can see (which means there is a lot more I can't see), you probably need at least a 1/2 tub IF everything from underneath your seats & forward is in good shape. There's a guy on e-Bay right now (RealSteel Cruiser Parts) that is selling rear steel 1/2 tubs for ~$2K and while you don't get the stock Toyota looking ribbed panels it seems to be a good price for steel.
A second option is to just get some large flat plate steel sections and weld them in right over the existing floor...but this just prolongs the inevitable. A third option is to try and do some patch work a little at a time.
Be careful cutting and welding on that rear bed floor!!! Your gas tank bolts right up against the underside of the floor!!! If you are going to get into working on this area, I'd remove the tank from the truck. It requires dropping the driveshaft, loosening the tub mounting bolts, and a lot of wiggling the tank around to get it back but it can be done without removing the tub.
You might want to seriously consider coming and getting the remains of my old '79 FJ40 tub floor...yours FREE for hauling it off...there are decent sections of OEM floor metal you can cut out of it and patch into your tub. Also I have rear quarters, rockers, cowl/firewall, and dash sections that can be re-used...will make you a deal on any of this other metal.
If you repair it, it will be a LOT cheaper than a new tub...but it will take time and effort on your part plus learning how to weld.
Let me know...