There was a Posi option for the HO33 differential, it was offered from '60-'62, possibly some '63a might have used it too, as the code is listed for '63. They are very rare, though. So far, in the almost 20 years I've looked for one, I've only come across 2 of them. One wasn't for same after I noticed the Posi tag (seller changed his mind), the other was just the carrier NOS that I found on Ebay. I don't believe the carriers will directly swap in the 3rd members, some of the internal parts have different numbers. The entire 3rd member will swap out, though.
Given the rarity of the oem Eaton Posi for that axle, llus the rarity of the Spicer Posi used in '59, the best bet if you really want one would be to do what Jerry suggests and find a later model 6 lug axle.
