I did more reading, and best I find is that manual or power steering boxes can interchange from 1963-1972 for A, B, & E bodies. That is because they have the smaller 1.125"D output shaft (sector shaft where Pitman arm attaches). There were internal differences such as whether bushings or bearings (as 65 Dartman says), but that also varied depending on A or B body and 6 or 8 cylinder. Pitman arms varied much more by body and year, but the gear didn't vary. Pitman arms might have also varied between manual or power in some years. If you get a compatible box from a bigger vehicle, it will be more rugged internally.
In 1973, both manual and power gear boxes changed to the larger 1.25" D sector shaft, which was always used on C bodies. Thus, I can use a 73 power gear on my 65 Newport (but hose fittings differ).
One guy vehemently claimed that manual boxes kept the small sector shaft in 73-76, but many others claim not from personal experience. I didn't find if early C body manuals had the large sector shaft too. C body manuals are fairly rare.
Vendors should do such research before they advertise. They put all that on each individual buyer, which isn't "value added sales".