Back when I had to patch floors in several old GM trucks and K5's I had,I at first only used pop rivets and self drilling screws to attach the "new" metal...then I finally broke down and bought a set of small oxy-propane torches ,the kind in a carry about pouch ,to do brazing and enable me to cut metal,bolts,etc..
Those torches enabled me to do a lot more repairs myself more easily..
Back them MIG welders were just starting to become popular and were still very expensive,so I brazed much of the sheet metal patches on after using some screws or pop rivets to pin the metal in place..
I used free sheet metal I got at the dump from old washing machines and other appliances,furnace ducts that were galvanized,and even filing cabinet metal, all found their way into my truck's floors..
Old above ground pools had nice thick sheet metal walls with epoxy coating and "ribs" stamped in it that make it really stiff ,that stuff works well for making custom floor patches..
Of course the torches had to heat the metal cherry red to braze it,which started the undercoating and seam sealer on fire,I had to keep looking under the truck to see if any was burning and douse it with the hose..
Once when I brazed a patch on my van's rear wheel arch,the inner wall started on fire,seam sealer ignited and the whole van's interior was covered with 3/8" plywood with foam backed carpeting glued to it,even on the ceiling..it could have roached the whole van if I didn't have the hose handy,or stopped brazing to see the smoke coming out from inside the van..
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A MIG welder can make a weld so quickly,the metal has less time to get hot away from the weld,so it is a lot safer than brazing with a torch..
Also if you weld or braze galvanized ,be aware the white smoke the zinc makes when it burns off can make you very ill,the "zinc chills" are no fun,I got them more than once despite doing the work outside with a fan blowing the fumes away from me--some always gets ya..
Another thing I learned the hard way,if your going to use screws or pop rivets and want to put some kind of sealant between the old floor and new metal,don't use RTV silicone or caulk,it releases acetic acid (vinegar) when it cures,which will rot the metal in short order...
Use body seam sealer,or butyl rubber caulking intended for gutters & metal buildings and roofing instead--I did use roofing tar a few times,but that pretty much kills any future repairs from being made,it'll burn like crazy after a welder or torch gets it hot enough..it also stinks,and takes forever to "cure"..
