Hey Milo..... Great tip. I never thought of that. I usually just do it by eye, and them are a real pain for me. Thanks for that pointer, I'll use that at the next opportunity, but I thought he was referring to the upper body line.
I personally like to work as much of the original body line out with the rest of the metal by, if possible, going inside with a straight object and pushing or knocking it out. You can also pull it out with the use of a stud gun. They are available from Harbor Freight for about $100. That not only gives you a reference to work with, it requires less filler, keeping in mind that body lines are targets for door dings and such. Filler doesn't hold up well in those situations. It is very likely that you will need some filler after that, but the amount is reduced. Other than that, the advise given above will help you get it done.