If you're going to make a spacer, you'll need a lathe, not a milling machine. The diameter and concentricity of the center hole is important. Start with a big hole saw with the proper thickness of spacer material mounted on a face plate, and finish the hole accurately with a boring tool.The rest of the shape of the plate can be done with a scribe and a bandsaw once the plate fits the bearing retainer properly. Get the center hole right, clamp it to the transmission, and use a transfer punch to locate the bolt holes. Then scribe a line around the transmission ears and cut the plate to fit with a bandsaw. Simple and easy!
Don't waste time trying to change the center section of the rear axle. If you do that, you'll need to lock the pivoting spring mounts some way, and welding cast parts with rubber bushings inside them is a pain in the rear end (yours, not the truck!), and rarely works for very long. Swap the whole housing since you've got it already.
Jerry
