Early A-body trivia!
I would think you find that lead on the aft part of the car....where the roof meet the quarter and don't forget the battery! Paint/coating....lots and lots of body sealer on the floors, trunk and inner fenders....it all ads up and was that wet or dry weight on the paint....and this day and age, you might have lost a bit of steel weight due to tin worm activity....but I am sure it is more than offset by rat turds and nesting.