64's used a gray casing, the ivory casing was used on 63s and 65 up used the black one.
The Chevrolet parts book lists the 64 (and 63) cable at 56 1/8" long. A repro of 56 or 57 inches should work fine. The 65-67s use a longer, 70" cable, because it was routed differently.
Is your car a 4 speed, if so, just make sure you have the original trans and tail shaft in your 64. On 63-64 4 speeds, the cable goes in the left (drivers side) of the tail shaft. On 65 up Muncie 4 speeds, it goes in the right side. If the trans or tail shaft have been changed, you'll probably need the longer, black 65-67 cable.