Will an 84 Camaro rear end fit a 64 Chevelle?

No, a 1984 Camaro rear end will not fit a 1964 Chevelle without significant modifications, and likely isn't practical. The mounting points, axle width, and spring perches are completely different. The 1964 Chevelle uses a leaf spring suspension, while the 1984 Camaro uses a coil spring suspension. Even if you could somehow adapt the mounting, the axle width is almost certainly different, requiring extensive fabrication.