Your rear is probably a 4.57. You could do a pumpkin swap with a late 60's up to '72 C20 and get a 4.10. However, with a 1:1 final ratio tranny, it's not super beneficial. You're going thru things I went thru a couple years ago trying to figure out what to do about driving faster than 60 mph. I opted for an overdrive transmission and kept my 4.57 rear. An OD tranny is going to get you the best gain. If it's just an every now and then driver, find a 4.10 rear and put 215/85/16 tires on. You'll be able to drive 65mph.
If memory serves, the model of the rear is H-072 or H-071.
-David
