I think the original rear end would have been an Eaton HO72. The stock ratio was 4.56, but it was also available in a 5.15. 1959 was the first year that limited slip was available as an option.

It will not be hard to find a replacement original with a 4.56 (either an Eaton HO52 or an HO72), but as stated above, this will give you a very low top end. The highest ratio you will be able to convert to is 4.10. These were available in the same Eaton rear end on 1967-72 3/4 and 1 tons trucks, mostly with automatic transmissions. The 4.10's are getting hard to find.

A 4.10 versus a 4.56 will give you a 10% increase in highway speed at the same RPM.

Matt