4.56 was the standard gear in the 3/4 ton for that year, with 5.13 as an option. The only definite option for better gearing is to find a 4.10 third member from a '68-'72, thous they are getting hard to find. There is a company claiming to have a 3.90 gearset, they still list them on their site (I don't recall the comapny name), but I don't think anyone has been able to confirm if they are for the Eaton diff or a Spicer.
A transmission swap to one with an overdrive gear may be a better solution.
You mention it being a posi, if so, it may not be the original differential, as GM didn't offer a posi in the HO 52/72 axle as far as I know. They did offer a Detroit Locker as an option in the later '60s, though. You migth need to confirm what differential it is (Eaton, Spicer, Corp 14 bolt), as this may open up more gear choices, especially if it's a Spicer.
