have the 8.5 corporate 10 bolt, with a 3.42 posi and like it alot. Made a huge difference from the 2.78 open I had prior. I am running a 400 HO, but the engine was only at 8.2:1 CR. Still had plenty of pop when I wanted it. I am now building a stock 400 HO plus a little, and it should be even better.