Make it a big block 4 speed sleeper. Put a 440 with a 65 or newer 4 speed transmission and a set of dog dish hubcaps and go out and annihilate the camaros and mustangs. You'll need to swap the /6 k-member to a V-8 k-member, 273, 318, 361, 383, 426 wedge cars all used the same k-member, /6 k-frame is /6 only.

Also cars like your Belvedere and other 1964 B-bodies like Savoy, Fury, Sport fury, and Dodge Polara's came with the 8 3/4 rear axle. Which is a good axle except for the tapered end axles, but you can get a new set of axles from Moser, Strange, Mark Williams, Yukon, etc... any number of companies. Swapping to the later bolt-in axles makes swapping gears and doing any kind of brake repairs a lot easier.

Is your car a pushbutton automatic, 3 on the tree or four on the floor?