Yeah, nobody makes a ring/pinion for that rear. There was one guy a few years ago in PA that made a 3.90 but, it was VERY expensive and he died so I don't know if they're still in business.
By far the easiest thing to do is what 64fleet said, swap out the center for 4.11 and do a 215/85/16 tire. Your gain will not be as good as a tranny swap but, easier and initially less cost.
A .72 (or better) OD tranny will get you to 70 mph, no problem, with the stock rear. An S10 T5 with a .72 or a WC T5 from a Camaro/Firebird (bunch of these have .63 OD) would be the way to go.
I installed a .63 in my C20 and I tach 2600 at 80 mph.
-David
