Quick: What was introduced in the spring of 1964, based on an existing chassis, and was totally unlike anything else at the time?
That’s right! It’s the Plymouth Barracuda.
The commonalities with the Mustang are quite evident, as Paul covered here. Introduced just sixteen days earlier than the Mustang, the Barracuda was based on the Valiant and was offered in both six cylinder and V8 versions.
Totally unlike the Mustang, you could get a four-speed manual transmission bolted to your 225 cubic inch (3.7 liter) slant six. We are still talking about the wonderful ruggedness of the slant six in various articles at CC; when was the last time you read anything here, article or comment, where one was waxing poetic about the various six-cylinder engines found in first generation Mustangs?
The Barracuda also offered the 2+2 seating (or perhaps 2+3 based upon the advertisements) that simply wasn’t available on the Mustang until the 1965 model year. The Barracuda also offered an unusual for the times degree of utility that could be traced to its Valiant DNA; fold down the back seat and you had nearly six feet of flat floor for hauling plywood, making out, or whatever else came to mind.
My nomination for the 1964 CCOTY is the Plymouth Barracuda. Sometimes the trendsetters get overlooked.