I put 3.91s in Vixen for a short time. I really enjoyed it behind the hot slant 6, but with 13" tires it was buzzin goin down the road, so I put the 3.23s back in.
thats probably. what Ill do. 3.91 in my 70 Dart. I also have a 741 case 3.23 and a 2.94.
3.91s for a while, 2.94s long cross country trip
3.23s rathole spare stash