That spring isn’t for any specific truck, it’s just the AG130 spring. You have to move the rear hanger to make it work on any truck that didn’t come with the AG130. The pics of the red axle is a W900 with the AG130 spring and House of Air bag bracket. There isn’t anything good that’s a direct replacement.
Your not going to get any info out of Stengel bothers because like any dealer they just sell parts. They’re clueless on anything that’s not OEM to a specific truck. You just need part numbers for the AG130.
Now friend that installed the AG130 on a 379 tried to use the short hanger that was originally on the rear to lower the truck (or keep it the original height) instead of the long ones that are on the AG130. Didn’t work, you have to use the long ones. So if you want the truck to stay at least the same height or lower you need the 5” drop axle. That truck had the 3” so they just set the ride height lower at first. Ended up swapping for the 5” drop and it is much better.
That’s the long AG130 hanger. Old one was removed, holes welded shut and drilled new in the new location.
With the 5” drop axle.