Just from a safety standpoint, I would remove the gas tank unless you can confirm that it's well sealed and there are no fumes that might ignite.
As far as grounding, I don''t believe you really need to do that independently of the ground cable to the welder. You truck (if basically stock) doesn't have any electronics that might be affected by welding.
Read through MPandC's Tech tip
here for some good tips on mig welding sheet metal.
