i would also suggest you leave a gap the size of your welding wire size if you make it perfect to the size of the hole with no gap , you will grind through the weld 9 times out of 10.
not to mention, if you don't leave space approx the width of the wire, during the weld process the two steel plates will push themselves apart, possibly affecting any body lines that may be close by.
don't know if you are familiar w stitch welding, this is often better then running long beads, allows time for cool down, and when there is nothing structual at stake, it only needs to be strong enough to hang paint off in the end.
oh, often usefull to have an air line near by to cool the weld spots down as you go.
man, serious, now that i think about it, its easy if you know what you are doing, but there are a lot of variables that can fuck you if you dont.