Well, im in the same position you are. Big hole under the battery, never welded before. But from what the professional welders tell me butt welds are better to lessen the chance of moisture seeking little crevices. Make a patch panel then tack it to see if it fits correctly, then weld a small section here, then on the other side, then the other to prevent warpage. Then after all the welds are met and finish put a seam sealer if applicable. They all tell me that you just dont want water to have a place to sit down because it'll rust out again.