LEAD SLED

Originally, the term 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 man-made fillers (such as Bondo), the term has come to mean a post-war, large (full size) American car of the 40's, 50's and early 60's (generally up to (but not including) the 1965 model year). It has also been used as a nickname for a variety of US military aircraft, including the F3H Demon, F-84 Thunderjet, F-105 Thunderchief, and SR-71 Blackbird.^[1]
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.

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References

References

The Auto Channel - Car Speak-To-English Glossary of Terms
Lead sled is also a common term for a sled (for use on snow) made of lead, but they usually are hard to pull so production stopped in the mid-70's.