My suggestion would be to replace the entire rear axle with one from a salvage yard. If you aren't in a hurry, you can probably find a good deal on one with the ratio you want. I'm guessing that an entire axle assembly isn't going to be that much more than a third, and will save you the trouble of switching it out yourself.

As far as the ratio, you've got to tell us a little more (tire size, auto trans or manual, existing gearing). The lower the gear ratio, the better acceleration you'll get. But you could end up running 60 mph at 4,000 rpm if you go too low.