Arc has no monopoly on blowing holes....witness my MIG welding to prove that ! It does help[ to use thinner wire,like 0.6 and up the power and wire speed to cope with thicker metal and weld penetration, rather than using thick wire and trying to turn the machine down. I suppose this would mean thinner rods and so on with arc. I used to work opposite a bodyshop whose owner had started out welding up classic cars on his drive with only a stick/arc welder. He told me that this was perfectly do-able and one could get very good results that way. Mind you, I have seen vehicles welded with all kinds of gas and electric processes and even brazed and lead soldered, all to a very high standard. Note : "seen"....all that is WAY beyond my capabilities. Practice,practice practice is the best advice. I always seem to have to re-learn the finer points as I work,so by the time I am finishing a job I am at the standard where I last left of and hence the work is ugly bar the last inch or so. This could be avoided by practice on non essential bits of metal,even fabricating non-vital bits if you cant justify the time and expense of pure practice for practice sake.
The other vital part is good thorough preparation. It will save you much welding time and frustration if you really go to town on the prep. So many times I have taken a chance on iffy bits of metal on the car and ended up spending hours putting on extra patches. Cut out any suspect stuff. If you are worried about the cars structural strength, clamp in some temp supports. Use a nylon mesh disc to get the paint off then a flap disc to get bare metal,wire brush cups tend to polish to a surface that LOOKS like bare metal but isnt good for welding.Use thin ,1 or 2 mm , cutting discs to take out bad metal.And always remember to bare a few good earth attachment points while you are at it.
Clamps and cramps are another subject that needs attention....get some good,metal sliding G cramps (they are CRAMPS not clamps!) and some of the excellent mole-grip based clamps,they can be had in sets or individually for cheap.The better you buy,the longer they will last,but a cheap set will do to start. The wide,flat ones are great,as are the arched ones with a gap....once you have used them on body panels you will wonder how you ever got along without them. Then magnets - very handy right angle triangle ones can be had for super cheap. Great to hold something while you get a first tack in.
The last thing I would stress is to get a really good earth clamp , or selection of them. The little spring clip that many welders come with are next to useless. They can be adapted if you cant get replacements, flatten out some copper plumbing pipe and drill through both side of the clips business end and drill&bolt on the copper plates. Use a thick piece of copper wire to make a loop between the bolts on either side. Always wire brush or scrape the copper before using to get a better contact.
If you can,replace them with heavier a duty spring clamp, a sliding screw up one like a G cramp and probably best of all....a magnet one.
Since you want to get the earth as close to the weld as possible, there are often times when a clip or clamp is not easy....thats when your magnetic attachment comes into its own. I have made them in the past out of ring magnets salvaged from microwaves stuck into a jamjar lid with a bolt to attach the lead . Whatever, you wont be sorry you got one !
On the area you are proposing to weld,be aware of any wiring that may run along the floor edge.You will obviously be taking all the plastic trim out of the lower pillar and rolling back the carpet. I have found my fathers trick of water soaked old towels or similar to be very handy when sill or floor welding - if anything gets too hot, whack a wet cloth on it for a bit. But DO be careful having too much water around when electric welding !
