Old Ray, the floor pan from the Mark is very likely galvanized, and the galvanized coating will be on both sides of the metal (the metal is either dipped or electrically coated (depending on how old the process was at the time it was coated), so unless you cleaned both sides, there is still a coating on the bottom side as well, if its not cleaned off, it will burn off and give off some gas that really bothers some people. Then you can add to that, when Ford welded those two floor pieces together, there is also likely some sort of sealer between the two pieces. That sealer would have to be cleaned off as well, or it too will burn off. The paint isn't helping either.

It also looks like your Plymouth floor pan has some paint, or other covering on it, that probably needs to be cleaned off in the areas your welding in, or it too will have to burn off. Someplace you need to get a ground to both pieces of metal to get a good weld, I usually grind a place clean to connect the ground clamp, and I usually clamp both pieces I want to weld together so I know I'm getting a good ground.

The 1st trick to good mig welds is to have clean shiny metal surfaces, top and bottom of both pieces. The 2nd trick is to have both surfaces you want to weld together tight against each other, any gap between them will either burn a hole, or the welding wire will have to fill the gap, either case results in a poor weld. Of course, the 3rd trick is lots of practice. Lots of light so you can see what your doing also helps, as does clean lenses on the welding helmet. Be sure the gas is turned on, and it can get through the nozzle, and that the tip isn't messed up slowing the wire down as it tries to get through the tip.

Probably not what you wanted to read, but I promise it helps with welding. Gene