You will find that most of the newer 9 inch rearends are wider than the stock 1/2 ton rear end. They may still fit, but if your truck is
lowered, you may have clearance issues with the fenders. My son has a 9 inch rearend from a late 60s Mustang that worked
great with no narrowing. I am using a 9 inch from a 73 Station Wagon, which is longer and I will have to shorten it ti fit.
There are also two different outer bearing sizes, so if you are looking for heavy duty, you will want the rear with the
larger bearings. Additionally, there are two types of splines, the 28, and the 31. The 31 spline rear end will take more
I should add, that the station wagon rear end actually has a larger crown and pinion, but the 9 inch pumpkin
fits with no problem. (assuming the splines match)
This link below, is an encyclopedia of the Ford 9 inch, and there are more links at the bottom
of his page.