No such critter as a "Y"-body, except the front-wheel-drive Imperials. The "Y" is the car line, Imperial in '64-'66 was a "D" body. From 1967 to the end of big-car Imperial, was a "C"-body.
It would be like calling Coronets and Chargers "W"-body cars; as we all know, they are "B"-body cars. They are the "W"-line.