I searched this very subject extensively sometime back as two of my trucks have corporate T-170's with 6.50's and I wished to find ANYTHING with a better ratio for highway driving.
Eventually I called one manufacturer (sorry, been long enuff that I cannot remember which manufacturer) to see what - if anything - could be purchased, or send my unit in for rebuilding with a better ratio.
The individual told me the best gear available for the T-170 was a 4.88 but that gear was impossible to find. His recommendation was to get away from the corporate rear as these were extremely limited with gear options, and to consider finding an Eaton rear as gear options were bountiful and I would have no problem finding a more highway friendly ratio.
Being in a rather rural area combined with a well defined case of laziness, I have yet to find an Eaton.
I hope someone with more knowledge on this subject will chime in and correct me if I was given inaccurate information.
Good luck on your search.
KC
