I believe boxed frame rails refer to frames that are in a C-shape configuration, where there is one of the sides that is open, and you weld another piece of steel (same thickness or a little thinner than the frame) to box the open end and help beef it up. (The only chassis's I can think of that have that style frame are trucks and full-frame cars, not unibodies) As you probably know, E-bodies are unibody construction cars, so the only 'open' part of the frame faces upwards and is welded closed by the floor pans. So unless someone has taken the time to add in tubular steel in the frame rails, your car wouldn't really be considered a boxed frame vehicle. (unless they do consider the trunk pan and floor pans as a part of the frame.)
