If the rear end is an Eaton (HO52 or HO72? can't remember which one is which) then you can get the third member from a '68 to '72 3/4 ton that used 4:11 gears and avoid changing out the whole rear end. It is a huge improvement over the 4:57 and makes around town driving far better. Use taller wheels and get more improvement. Then use some type of overdrive like the t5 or a gearvendors and you can cruise at about 65mph and spin at about 2000 to 2300 rpm's. Much greater driveability.
