posi rear end in '56?

[mark '87 930]

I have stock rear end in the truck with 4.10 gears. Is there a drop in pumpkin one can get to get a posi and retain the 6 lug? I though ti saw some posi units that bolt in from a Belair.

anyone know? or do you have to change the whole rear axle.

I want to retain the 6 lug and preferably not have to do any drive shaft mods (cutting or having to buy a new one.)