There are 3 different carriers used:
2.56 and 2.79
2.93, 3.08, and 3.23
then 3.36 and up
Auburn offers a new posi for the 2.93 to 3.23 ratio,
Eaton offers a a new posi for the 3.36-up ratio
I'd highly recommend contacting Jim Monzaz at
www.jdrace.comJim posts on several Buick boards, knows the Buick rears well, and will sell you the CORRECT parts at a great price.
His website is very informative as well.
A replacement rear would be the easiest solution. 64-66 Buick, Olds Cutlass, or Pontiac Lemans will bolt right in. The 67 rears use different rear springs (Easy fix), and 68-up are approx an inch wider. 12 bolt Chev rears are nice, but in demand(and expensive)