ok I"m going to have to get a piece of exhaust welded once my beast is running.. I heard before that an arc welder could damage o2 sensor.. is this true?

 

I've welded on cars many times and never developed electrical problems, but if you want to be on the safe side, you can disconnect the battery to isolate the car's electronics. Many people (and publications) recommend this, though like I said a lot of people don't bother with it and never have problems.