Elmo,
If you check at the top of this (and every

) page in the Stovebolt Forums, you will see a link for our
Tech Tips section. Once there, you can scroll down through the many helpful articles to the "Axles" section to find an
article on upgrading the 3/4 and 1-ton rear axle.
The truck in the article is mine. With the 261 engine, 17-inch wheels and the 4.11 ratio, I can hold 65 easily. 70 MPH will be more fun with tightened up steering (:) ) or radials ...
Before you get too crazy with ratios like the 3.73, you might want to be sure your other components are up to that. If you still have a 216 in your truck, you will not get the desired performance boost from a 3.73 -- the engine will be over loaded. From what I hear, the 235 does ok. The 261 is a nice combo with the 4.11 ratio, though (and taller wheels/tires).
Before increasing your cruising speeds, please ensure your brakes, steering, tires, etc are up to the increased demands of higher speeds.
Finding the center section/3rd member for the HO52 Eaton axle is getting harder, but not yet impossible, and replacing yours with another with a more desirable ratio is far simpler and easier than switching out the whole rear axle assembly.
Read the article.
Regards,
John