Originally, the lead sled referred to extensively repaired/modified cars due to the use of lead as body filler and was, for the most part, an insult.
Since the introduction of artificial fillers (such as Bondo), the term has come to mean a post-war, large (full size) American car of the 1950s.
A lead sled tends to have all extra side moldings shaved off and filled with the suspension lowered. Lead sleds are built for style, not speed. Some common lead sleds are the 1949 Mercury, 1949 Ford and the 1959 Cadillac.