Stockers have to use a corporate rear, so for GM cars it's a 12 Bolt for them. Some still use the 8.5. Mopars will use a 8.75 or a 60.

Super Stock can use whatever. I don't know of anyone who's used a 60 in an AH car for many years. The 12 Bolt center section is typically a lightweight car or a Comp deal where the last ounce of efficiency matters and frequent maintenance is part of the drill.