If your shim has a hole in the bottom, you clamp the spring down using large C clamps, remove the original center pin (you'll need to file or grind the top of it down and you may have to hold the old pin with vise grips), install the shim oriented the right way on the bottom, slip a new and longer center pin in and tighten it using some sort of thread locking paste. You'll need to trim it at the top for the frame bumper to fit. Leave about 1/16 inch above the nut and pound it down with a heavy hammer. You don't want there to be any chance it might unscrew. Then the whole she-bang sits on your axle and the pin fits in the hole and you're ready to go. You might need longer U bolts or you might not. 6 degrees is on the high side. If your shim doesn't have a hole in the bottom, drill one the size of your center pin's bolt section. If your shim has a U shape cut in the bottom, send it back for a refund and get one with the right size hole in it. Sometimes the shims will come with new center pins.
Please don't mickey mouse anything to do with that spring pin or the U bolts. If you've ever seen a situation where an axle became loose, unconnected or shifted on the spring, you know how dire the consequences are, how fast control is lost and how everything instantly goes to poop when the driver hits the brakes and they almost always do.
Jon
