Re: Fractory 12 bolt or aftermarket
You can get a complete Moser 12-bolt with sturdy 30-spline axles (c-clip), heavy duty pinion yoke that accepts a 1350 u-joint, and an Eaton Posi with gearing of your choice for about $2200. That's all new stuff which will be much stronger than original for about what a used 12-bolt goes for, with who knows what's in it.
If you're into performance, the choice is easy; aftermarket. If you're into numbers matching or originality, go the original 12-bolt route.