Welding of 3/16" to 1/16" steel can definately be done with the equipment that you have. I have similar equipment and electrodes at home for playing around with. I have tried both the .035" and the .030" self-shielded FCAW electrodes in mine, and have had sucess (after many trials and tribulations) with both. The .030" electrode would be the easier of the two to weld the thin metal with, but it could be done with either. The key is machine setup and technique. I don't know what amps or volts I am running because the silly knobs are just labeled with letters and numbers that do not indicate any actual values, but I tend to run my machine near the top to middle of the voltage range, and the bottom of the wire feed speed range to get the best results on thin material. Possibly more important though is the welding technique. Make sure when you are welding, you arc is concentrated on the 3/16" thick material. When you puddle starts to build up, and you are sure you have fusion with the thicker material, move the arc across to the thinner material just long enough to get the bead to wet it, and then move the arc back onto the thicker material. If you can get some scraps to practice on, you can play with machine settings and technique before blowing holes in your floorpan.
Hope this helps
If at all posible go to .023 solid wire with CO gas.The CO gas helps in windy conditions and the .023
wire is great on thin metal,I have a welding business and I never use flux core on thin metal.Flux core
seems to get much hotter and warp more than with gas.This is just my opinion but I hope it helps you.
My heartfelt thanks to all that replied. I did get a bottle of 25/75 plus .023, .030 and .035 solid wire. With the thinner metals I did practice on a few scraps and found the settings that worked best, giving good penetration without warping or burn through. The car passed all inspections as was/is certified by the sanctioning body. Thanks again for all the help!