It seems there are a few of us working on floor pans or other welding challenges I have had replacing the long fronts in my 67 Mustang is getting the seams to together, as well as occasional blow through when encountering thin metal or wider gaps than planned.
Copper/magnetic heat sinks in a restoration catalog that will prevent molten metal from dripping or blowing through the project which normally will prevent this, so I gave it a try. This gadget is not too expensive and it works ok, if both edges of your project are flush or flat. If not, there is too much gap between the work and the copper to be effective and it can make things worse.
I gave this some thought and expanded on the magnetic heat sink idea. I went to the local home supply store and picked up a short piece of 1”X 1/4” flat bar, a piece of ¾”X 6” copper pipe, some #6 X ¾ machine screws and nuts, and a couple of #10 fender washers.
Back in the shop, I first flattened the copper pipe in a vice to keep it uniform and flat, then I banged it a few times with a ball peen hammer to flatten the edges. I cut my flat bar to a six inch length, then scribed a line down the center, and marked to hole locations one inch from each end. I punched the holes then drilled a hole in each end. I then ran a #6 screw into each hole, and tacked if from the head to the flat bar with the MIG.
I then transferred the locations of the screws over to the centerline of the flattened copper, and drilled a corresponding hole into each end of the copper. Once all drilled and welded, I slipped the copper over the two studs. The ¾ copper flattens nicely to a bit more than 1”, giving the flat bar a nice edge.
Once the copper sink and backer are fab'ed together, I went to the seam in the floor pan (transmission tunnel) line and laid the sink up next to the area I wanted to pull together, and marked the bolt holes with a sharpie. I then drilled a 9/64 hole just below the seam in the good floor pan material. From under the car, I pushed the two studs through the holes, and then from inside the car I placed a fender washer and nut on each stud, and ran them down snug until the seam was pulled flat and the copper was snug up against the metal.
Then it was just a matter of tacking and welding the seam closed. When done, unscrew the nuts; drop it out, weld up the holes and your done.