Use .030 wire and it's not gonna be an issue. I had trouble when I was using .035 wire, but .030 and it's butter AS LONG as you sand/grind ALL
of the sealer and glue stuff off.
This definitely isn't a textbook weld, but it's 3/16" to the unibody, not a problem.
edit: I've seen Kraqa's article about welding on unibodies all over the place. He's actually local to me in the Lower Mainland of British Columbia, active on the local forums, etc etc. He does phenomenal work. His work is clean, well-executed and well thought out. He forgets more stuff in a day than I know about, period. But every time I see that article, I have to disagree about the way he welds on this bracket:
Stitch welding is a great idea. But it doesn't need to be taken to that extreme. Three longer welds on the top side of that piece would have been enough, properly spaced, not 5 miniature welds. With MIG, that's just a bunch of cold starts to me at the beginning of each weld.
Sort of how I did this reinforcement plate for the shock hoops in the rear:
(again, my work isn't textbook either. I eyeballed the lengths of the welds as opposed to marking them off, for one.)