Making rubber bushings
First, order some urethane casting compound. Devcon Flexane is a good choice, or, a more generic, less-expensive product, such as part #8644K11 from McMaster Carr. This can be used to repair seals, make molds to cast ceramics, etc. It comes in two parts; you mix the two together, and then pour them into your mold. They take 48 hours to cure, and there is no baking required. Because this compound only shrinks 0.0005" to 0.0014" per inch of thickness, there is no need to worry about making your molds oversized.
Molds can be made out of steel tubing -- you can use the steel tubing that will be housing the bushings; that will ensure that the bushings are the correct diameter. Bases can be turned out of HDPE plastic, so the tubing presses into it, and then you can drill a hole in the base for some 3/4" nylon rod used as a center core to leave a hole for the bushing spacers.
Here they are popped out of the mold. The two longer ones will get cut in half and be inserted into both sides of the rear shackle mounts on the frame. The smaller ones will get bushing spacers in each one that will go into the eyelets of the springs, and the rubber bushings will be housed in the steel tubing welded inside the shackles.
For the bushing spacers, you can use some tubing with a 9/16" ID and 3/4" OD. That way, 9/16" bolts would be a nice fit inside of them, and it fit the 3/4" ID of the bushings. Turn down one end of each spacer so that they will press into the eyelets of the springs.
The bushings will then slide onto the other end of the spacers, and will be housed in the tubing used for the molds, which will be welded to the insides of the shackles. The bushings will be able to rotate inside the tubing and the spacers will also be able to rotate inside the bushings when the suspension moves.