I'd say go for a rebuild this way you stay away from the 'mass roduced' ready to bolt in units. The previous owner of my 63 had the diff rebuilt for $1200 but he had new Richmond posi unit and gears with new Moser axles put in which explains the amount he spent...ther's no reason why you should not need to spend more than $600. You could always get the internals from a 68-72 12 bolt and swap it over