It's all about tire diameter. Your 235/60/15 tire diameter is 26.1". The online RPM calculators show 2074 RPMs at 60 mph with a 3.73 rear and 0.72 OD gear. If you swap to a 3.08 rear then you will loose some power off the line but get better RPMs at highway speeds. Your new RPMs with the 3.08 rear will be 1713 at 60 mph.

1965 Chevy C10 Shortbed 1/2-Ton Stepside
Truck College
In the Stovebolt Gallery
In the Project Journals Forum
The Build BLOG on Weebly
T5 Transmission Build page