I would think that as long as you have your grounding cable connected pretty close to the area you're welding on, the current shouldn't be flowing through other areas of the frame/etc, as it will take the path of least resistance to ground. Hopefully someone else with more welding experience will chime in to confirm this or tell me I'm an idiot
Just to be safe, however, I would probably disconnect the battery, and use welding blankets (if you can) to keep the sparks away from any electrical equipment. Also, make sure you're not welding near any fuel lines.