Depends-are you trying to save the paint underneath? If you are trying to save the paint heat with a heat gun or wife/gf`s hair dryer otherwise razorblade. There are also decal removers that spray on or wheels that "buff" them off. http://www.tcpglobal.com/autobodydepot/adhremove.aspx
Hair dryer followed by a bunch of adhesive remover. Should take all day if it's 40 years old. An erasure wheel may also work use this option at your own risk.
Heat gun/hair dryer for sure. Had to do my Ram years ago due to assholes steeling the bees and making a mess. Also used a good firm plastic putty knife, Good luck