In 1965 Plymouth once again made the Fury a full-size car, and Belvedere became the intermediate size offering.
The 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 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, 2012