A heavily customized car that used lead as body filler is called a lead sled.
The term describes a style of American custom car popular in the late 1940s and 1950s, especially large Ford, Mercury, and other V8-powered cars. Builders reshaped the body, lowered the suspension, removed trim, and smoothed seams and handles to create a sleek profile.
“Lead” refers to molten body solder, which was flowed over shaped metal panels and filed smooth before painting. “Sled” alludes to the car’s lowered, heavy appearance. Modern polyester fillers and fiberglass largely replaced this hazardous technique, but the name remained part of custom-car vocabulary.
A lead sled is not simply any old or slow car. The phrase usually implies a deliberately customized body, often with chopped, channeled, frenched, shaved, or dechromed details. “Rat rod” is a different style, generally emphasizing a rough, unfinished, or deliberately weathered appearance.