hey bro couple of suggestions for you
1st check and see what the sheetmetal thickness the floor pans are. I think the 12 ga (.104) sheet metal is right around the proper size, so what you have should be good. 10 ga ( .135) might work also but it might be too thick if you want have it flush with the existing floor. btw dont buy your sheetmetal at Homedepot or Lowes it is way overpriced for what you are getting. find a metal supplier in your area. the price of what you paid for a small piece at HW you can get a very large sheet. they might also have a scrap section that they practically give away for cheap.
the next problem i see is you are either using a Flux-core welder (gasless) or you ran out of gas (argon/CO2) on your mig welder. if you are not using gas i strongly recomend getting your welder setup for it. The quality of your weld will be day and night and the results will be ten times better.
if your are using flux core, your welds might improve slightly with practice but i really recomend uprgrading to gas if possible. if not then practice on some scrap first and see if you can dial it in until they look reasonable. and if the welder is at its lowest setting and still blowing through it could be your welder. i know Harbor Freight sells the cheap welders and they click on preset heat settings so makes it hard to dial it in correctly, you need a welder that doesnt click (variable) on the dials. btw take a pic of your welder so we see what you are working with. welding is not hard once you get the hang of it and use the right tools.
sorry so long but hope this helps. keep up the great work!!