what rear would you go with

I've used the 3.23 gear set and the 3.91 hear set.

The 3.23 is a great all around gear which may not be slammin you in the seat off the line but it's still a good gear that you can still blow people off the line with, and your highway rpms are much more friendly with the 3.23s.

I currently have a borrowed 3.91 set in the car which is fantastic off the line and burn outs, but the gears just plain suck on the highways especially when a mustang passes you and wants you to give chase but you can't because you are already turning 3500 rpm cruising at 70 and you don't want to blow the motor up just trying to see who has the biggest balls.

If I had a choice I would go with a 3.73 gear or a 3.55, but For highway cruising and and all around great gear the 3.23 is hard to beat especially if you want to get up to 90 mph in a hurry to show the mustang who's boss without redlining the motor.

I have a 742 case 3.23 set with the clutch style sure grip in excellent condition that is dry, quiet and not sloppy that I would trade even swap for that 3.91 if it's in great condition as well.
Just something to think about.