Depending which spot weld drill you have I'd say go easy on the 1st one as the drills work better once you've taken the edge off them, they can be a bit too keen when new.
Also be carful if the 2 panels 'shear'against each other, if you trap the drill it can snap.
Take a little out and try to carefully prise the panels apart until you have some idea of how deep to drill, it's very hard welding in the new panel if you've left the old bottom panel as thin as tissue paper.
Put new welds in place of the old ones if possible and use a good weld through primer before you start.
take your time!
Good luck :D