This post didn't get many takers, hopefully that's an indication of the general dependability of the GM truck 12 bolt.
Anyway, for posterity's sake, the solution I came up with was to swap in another 12 bolt from a '69. The control arms mount up fine and the width of this rear is correct, but one problem you'll run into is the tracking arm. Apparently they changed up mounting locations in 65 so you'll need an adjustable tracking arm/panhard bar that is compatible with the 64 frame.
The more you know...
