Sounds like it may have a '71-'72 axle in it. That axle is more or les a direct bolt in, housing and all. There should be a code stamped on one of the axle tubes that will give the year and original application.
As far as the diffs go, Chevrolet trucks used the truck style 12 bolt from '63-'77, some 4x4s used them later than that, up to maybe '81 or '82. And being a posi doesn't necessarily peg it as a 30 spline, GM did offer a Posi in both styles. But being a 5 lug is a good sign that it's a later unit. A good way to determin if it's truck or car is to look at the wheel bolt circle. Trucks are 5x5, car will be 5x4 3/4.
