1963 Plymouth Belvedere: Starting with the 1962 model year, all full-size Plymouths were "downsized", with more compact outside dimensions. American car buyers at the time were in the thought mode of "Bigger is Better," and sales of these models suffered. However, the big block V8-engined (426) Belvederes proved to be a terror on the track, and became a desirable drag race car due to its lightweight body.
1963 and 1964 models used the same unibody platform as the '62s, but were restyled to look longer and wider.
The 1964 Belvedere (and corresponding Fury hardtop coupes) featured a new "slant-back" roofline that proved to be popular, and sales improved significantly over '62 and '63.
The 1964 Belvedere is also notable for being the car used to introduce the 426 Chrysler Hemi engine, which used a canted large-valve arrangement. This was such a significant high-RPM breathing improvement that Hemi-equipped Plymouth Belvederes won 1st, 2nd, and 3rd at NASCAR's 1964 Daytona race. One of the winning drivers was the famous Richard Petty.
Source: Wikipedia, 2013