Rear Gear Change

[6xchevy]

My 68 shorty came with a 6 cyl and 3 speed on the column and either a 3.70 or 4.11 rear. The 350 in it now with the same drive train is not happy at 55 or 60 mph, would it be a better choice to change the ring and pinion or to change the entire rear end to get a highway friendly set up? What rear end assembly would be the easiest fit? I am thinking about going to something in the 3.08 - 3.36 range. Thanks for any help.