Is it a clean break or has it chewed up the crack some? Can you get it clean enough and prepped in the car to weld in the car? Any issues getting the mig torch head in there and will it be a difficult weld in terms of the position? If it is possible to weld in the car I'd remove any ignition boxes, and voltage regulators or other electronics in the car lest the weld current damage them. I fried a voltage regulator on a mower welding up a part with a mig welder.