maybe some E6013...3/32". i'd say 7018, but that might penetrate too much and you'll end up w/ holes all over. 6013 is a light penetration rod specially made for thin sheetmetal. use a single pass, all the way around should be sufficient. if the mount is sorta like a "U" shape, you shouldn't have to weld on the outside AND the inside...only welding it on the outside should be good considering the thing was only tack welded coming from the factory.
mac, personal preference, I would use a 6010 3/32'', seems to work easier for me than 6013, never was too crazy about 6013, maybe it's because I don't do a whole lot of work with light material, but I fixed up an old Vega for a lad a couple years ago and he's still tearin up the asphalt with it, and he had no problems with it, used 6010. I would say experiment with what you can get your mitts on, and what works best for you on the scrap your play with use that. Single stringer bead will work fine. You really wouldn't have to go too crazy with the amount of weld you put to it, unless your gonna hammer the car, then you got to operater proof it, which is a whole different ballgame. Have a blast and God bless. Oh and by the way Honda steel from what I've heard ain't all that great to work with. Hope it goes well, have a blast and God Bless. J Krout