No 12 bolts from the factory, but Tom's Differentials and now Strange has conversions($$!). They were a 10 bolt from 63-82 17 spline pinions and side gear/axle stubs for the IRS. So a 30 spline 12 bolt conversion is a good upgrade for high HP motors. Why I as wondering how different the posi is in them and ours are.
Wonder if an 8.5 GM 10 bolt posi would fit in our pumpkins? 28 and 30 splines for axle spiders available.
Some of the later Vettes used Danas.