I am a sheet metal fabricator, we weld galvanized daily. The key is ventilation. Excessive exposure gives flu-like symptoms that will pass after some discomfort. My point is, don't be afraid to weld galvanized, if the metal on your bus is indeed galv. If you don't want to or can't weld, don't rule out good old pop rivets and some butyl rubber caulking. Rivets don't look out of place on a bus. Butyl rubber accepts paint, stays flexible and will not attack the metal as some caulks can. Hope this helps.
79 International Wayne