That appears to be the GM Corporate Eaton HO52/HO72 style rear end. It is more than likely a 4.57:1 ratio. There is a huge amount of information on this site regarding that differential. To the best of my knowledge there is only one "higher" ratio available and that is a 4.10:1 from 67-72 trucks with an auto trans. It may be even more difficult to find a gear set for a "NoSpin" unit. You may be better off installing a manual overdrive, Gear Vendors overdrive, or an automatic overdrive transmission and leave the rear end alone.
Fred
