Metal work help!!!!!

In the pictures the metal laps around the curve which would indicate that the metal has to stretch in those areas to lay flat. I would as recommended by mustang42782 put some slits in those areas and that should allow you to push down the metal and hold it until it is tacked. In the other areas (if I can describe this right) take a piece of flat bar maybe a foot long and width of lets say 2". Place it on the metal that you need to go down with the 2" from the bottom of the floor pan to the top of the tunnel direction and the top edge at the edge of the new metal (where you want to weld at) and tack weld the lower edge of the flat bar to the pan. Now you can leverage the metal down and when it is tacked you just bend the other way and it will break off and onto the next spot. It alot faster than it sounds like and can be used on the bottom piece the same way after you have all the top tacked off. It works well but is not that easy to explain. I hope I have explained it ok but if not let me know. You mentioned increasing wire speed helped, start welding on a piece of scrap metal and increase your wire speed or slow it down as you weld and this will help find the sweet spot. I can do this and tell by the sound when its right. I would agree that a regular mig with tri mix would be easier and the metal has to be clean. Any weld contaminents will cause perosity in the weld.

To help with the burn through problem, I generally back the weld up with a strip of copper or aluminum. The weld material will not stick to these and will even allow you to close up holes. If you use aluminum it needs to be at least 1/4" thick if possible so that you can remove it easily as the weld may stick to it lightly but nothing so hard that you can not pull away the strip.

Scott...

The dirty surfaces aren't helping with your burn-though issues. Everything needs to be shiny and grease free. Also keep your heat down, go slower, and give the MIG time to do it's job. Or switch to a TIG. That's what I use for all auto sheetmetal work, heat stays where you put it.

As for the gap, like FE said, always tack the corners and a few stitches along the sides before running a full seem. But for now slitting that curve is your best option.

Thanks for all the help, finally got the pan welded in. The slits and self threading screws were the most help. I think my biggest problem was that I put the One Step on the old metal before welding and I should have waited until afterwards, the MIG didn't really like the rust converter.