Who swaps them to a 12 bolt and why?
A STOCK 8.5" 10 bolt will take 1.50 shorts under a 3500lbs car. My old T-Type went 1.47 with over 100k miles on the stock diff. Please, go into detail about the weak point of the 8.5".
... and the racing comment, yeah, I've never owned a fast Buick and know nothing about them.