I converted a manual steering assembly on a 62 Impala convertable to a power steer unit out of a 80 Chevelle. I was able to utilize two of the existing holes, however I had to weld an bracket on top of the frame to accomidate the top bolt. The 62 chevy frame is the same as the 63-64 so you shouldn't have any problems.

There are now other options available, like a "500" box, that you could investigate. Eckler's has those available also. I believe there are some radiator clearance issues, though, if it is a big block car. It may be worth checking out.