Pull one of the stock axles and count the splines, they probally are either 30 or 35 splines, the 18 and 23 splines where pre 1970 Either way you can have new car type axles made for your rear end just like what came in the original cars Are you going to have it shorten to the stock width or narrower like a 1970 and earlier B body? That width might fit better Dana made those rear ends for all three car makers so they have parts to make any axle spline fit in the posi and open carriers I have swapped the spider and side gears from 23 spline to 30 and 35 splinnes to use the bigger axles in cars and trucks