I don't know if the Chevelle bushings differ dimensionally from the Impala. I do know that they are different than those that are used on the Camaro and Nova.
If you have a drawing of the bushings Energy Suspension can machine them out of a bar of round polyurethane, but then again any machinist with a lathe could do the same thing. Difference is Energy Suspension might want those dimensions to bring another product to market as big Chevys are growing in popularity. Once they create a CNC program they can make the parts on demand in small batches.