Before you choose something, call Doug at Quick Performance 515-232-0126 (
www.quickperformance.com). They mostly do 9" Ford rears and have bolt-in units for your Olds, various prices depending on what equipment you want. At least you would have the strongest starting point for any future upgrades if you try drag radials etc. in the future. I wouldn't put $1200 into a 10 bolt. There are enough of them left in mild ratios in boneyards, and the only really stout and positive positraction is the Detroit Locker, which you can't get for GM rears but can for the Ford. Don't get nervous, Olds bought their HD 3 speeds from Ford so Ford is a legitimate drivetrain vendor for your Olds.