Stock 48-late 50 F-1 rear axle was 3.73. If you can find one in better shape, questionable these days, that would be quick and easy. 3.73 is really a decent ratio, 3.50 is about as low as I'd go, and it gets pricey to get below 3.73 (carrier break point).
If you want to go to a lower ratio, Randy's R&P sells the parts needed to put modern gears in your old case. But it won't be cheap. You have to replace the carrier, the R&P, and side gears, from what I've heard you're looking at an easy $500 in parts alone. And Dana's have to be set up carefully or they make noise, so mo' money there if you farm that out to a pro.
http://www.ringpinion.com/Default.aspx
I know someone with two 3.73's, condition "average" I'd say, but nowhere near you. There are likely plenty of them out there from guys doing 9" swaps. It would be well worth your time to open yours up and see if it just needs bearings.