Most people don't disconnect anything. I always disconnect the ground at the battery, but I don't think that is even necessary. Are you going to MIG weld it? Be careful with the heat and your hood. You may want to fix a wet towel to the top of your hood while welding to absorb heat and stitch weld depending on the clearance ( I can't tell from the pic) It would be easier to braze it if the area can take the heat.
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick