Lead sled

In automotive usage, a lead sled is a standard production automobile with a body heavily modified in particular ways (see below); especially, though not exclusively, a 1949, 1950 or 1951 model year Ford 'Shoebox' or Mercury Eight car. In the name, "lead" (as in the heavy metal) refers to the heavy weight of the body, and "sled" refers to the lowering of the vehicle, giving these vehicles the appearance that they were "slip sliding" down the highway.
Period auto body repair, by an auto body mechanic used to be achieved through a combination of re-shaping sheet metal using specialist hand tools and the application of molten lead to damaged body panels, fulfilling the role of more modern polyester fillers / bondo.
The same techniques were also used in high end low volume car production (coachbuilding) and adopted for aftermarket hot rodding body panel modifications.
Automotive usage.
In order to be classified as a “lead sled”, the vehicle was subjected to most, if not all, of the following body style modifications:
The entire process of removing badges, trim, and doorhandles was referred to as "shaving".
Grill modifications: the original grill was heavily modified, or substituted with the grill from a completely different make, model, and year car.
In the late 1940s and 1950s, plastic body filler and fiberglass did not exist. Instead, bar lead was used as a body filler. A true craftsman pulled and pushed out dents with body spoons, hammers and dollies until the sheet metal was as straight as they could get it. Any sheet metal that was still slightly wavy, the bodyman heated bars of lead and flowed the lead onto the body with an oxygen-acetylene torch similar to work done by a tin smith. The bars of lead were what we today call “solder” but were not the wire material we are familiar with today, typically sold for electrical or plumbing repairs. The lead bars or strips ranged anywhere from a quarter of an inch to one inch in width and several inches in length.
Lead craftsman call the process of melting the lead “running lead” and this is a highly specialized ancient trade passed from a master craftsman to an apprentice. An apprentice bodyman typically would remove the body part from the car and place it on a bench so as to have a fairly flat surface to flow the lead horizontally onto the body. In contrast, the master craftsman could control the heat of the lead in a vertical position without having to remove the body part, thereby saving time in performing the repair.
An apprentice bodyman most likely would have to grind and hand file the lead to a smooth finish for repainting. The master craftsman on the other hand did not have to grind and only had to hand file, if he had to perform any smoothing at all. The true craftsman controlled the flow of lead with his torch and most times could produce a satin finish without filing.
"Lead" refers to the body material used and the extra weight added by the repair material. "Sled" refers to the lowering of the vehicle, giving these vehicles the appearance that they were "slip sliding" down the highway.
As time progressed, plastics such as "Bondo" were introduced to the market. These plastic body fillers are easier to work with and eventually replaced the use of lead in body repair.
Some common late model lead sleds are the 1949 Mercury, 1949 Ford, and the 1959 Cadillac.
Other usages.
Among aircraft nicknames, "lead sled" has also been used as a nickname for a variety of US military aircraft, including the F-4 Phantom, F3H Demon, F-84 Thunderjet, F-105 Thunderchief, and SR-71 Blackbird. In particular these airplanes tend to be large, heavy or very fast. Despite this, the airplane's maneuverability is relatively poor. The F-105 gained this nickname during the Vietnam War. While the plane was fast in straight lines it was not very maneuverable, rendering it very vulnerable to enemy weapons.
References.
The Auto Channel - Car Speak-To-English Glossary of Terms