Barry, mine is a 1965 K1500 (GVW 8100 lbs) with the std D 60 and was a 4.56:1 now a 3.70:1 but still an open diff. Any chance you might have a posi unit (or even locker) in the area of 3.70 or lesser number / higher ratio? If you want you can contact me via direce email:
jim@james-associates.com when you have a moment. Thanks, JIm Hyde Phx AZ.