I just finished rebuilding the steering gear box in my 1965 Belvedere II wagon using a Gates 350510 kit from Rock Auto. $32.79. This kit included bearings, all seals, O-rings and packings.
The reason I rebuilt mine was the upper shaft seal was leaking and I could of just replaced the seal, but figured I would completely rebuild the unit. I have an original service repair manual and it helped. The instructions included with the Gates kit are not very complete. The next one I do, will be much easier.