Oh boy,oh boy, oh boy... Do you already have the patch? New or an old cut out. If you want a nice fit, try this:
Using some generic numbers we'll call the HOLE 12" x 12". Mark the floor 1" larger all the way around. Now the MARK is 14 x 14. Take your patch and square it up just within those marks, cutting as needed to fit. Tack it to the floor so it wont move. Mark the new panel 1/2" smaller all the way around. So now its tacked and immobile and marked for an accurate cut. Plunge cut the skinny cut off wheel and follow between the marks on the new and old panel cutting through both the patch and the existing floor at the same time.
Grind off the locating tack welds and debur.The new panel being cut with a 1/32" thick wheel and around the circumference will be 1/16" smaller and just about drop into position. Yeah... be careful on the piping and PB cable! Whats the rush right? Take a 1/4" ish piece of aluminum for a bucker upper and tack the patch in. Try to keep it centered for the gap. You can use a 2x4 to push up off the shop floor and push down from the top so its flush for a tack. The aluminum wont weld to the steel and will help keep the patch cooler too. It will also help protect the gas and brake lines and cable from spatter burns while welding. Make sure BOTH SIDES of BOTH pieces are ground shiny clean with some 36 grit discs. Any crud at all will find its way into the weld. Its called capillary action and will get sucked right in. Finish as desired.
Just weld in little dabs and move around so it doesnt warp. It'll even pass the judges! Bill