My automatic 727 backed 70 Duster has a 741 case open diff with 3.23 gears. I would like to replace the center chunk with something stronger & locked. Ideally a 742 or 489 case Sure Grip with 3.55s. I have a chance to get one with 3.91s but wondering if it will be too much for a mostly street driven weekend cruiser? Here in AK we don't have any real interstates like in the Lesser 48 states :poke: so highway cruising @ 80 for extended miles doesn't really happen, and most of my driving is around town. Will the RPMs be too high or occasional driving @ 65-70 MPH?
