Rich -
The stock rearend in your 3600 should be an Eaton HO52. There is also an Eaton HO72 available whihc is essentially identical. Chevy/GMC 3/4-ton trucks from 1967 thru 1972 used the same Eaton rearend except that some had 4.10 gears. I agree with JiMerit that it's a straight, simple, "2 beer" swap. You should be able to find a complete pumpkin with 4.10 gears for a reasonable price at:
www.67-72chevytrucks.com . You will pay more for the "No Spin" posi-traction setup.
Good Luck!
Dan M.
1949 Chevy 3600 5-Window
