I would almost recommend sheet metal templates tacked in place on the good parts of the pinch weld. THEN cut out the bad stuff and get that tacked in before cutting off the templates. That would serve two purposes, stiffening the opening and maintaining the correct shape. It would also give you something to clamp your patches to for welding. Cardboard is a little hard to weld in place.
That would only apply though if you're leaving the majority of the good metal in place, not for replacing the complete top.
Good Luck. Looks like you have a handle on it. Spending some time studying the situation is always good. Falls into the category of "measure twice, cut once."
