No aftermarket posi for the corporate housing. You might get lucky and find a 3rd member from a '60-'62, which was the only years thaat posi was offered in that diff, but it's very unlikely due to the rarity (I know, I've been looking for one myself). As far as oe posis in a '59, they used a Spicer 45 (Dana 44) rear end from a GMC. You cauls swap one of those in, it'sll have the right width, and posi units are available for it easily.
