I think I would skip the poly bushings, it seems as if they may cause an "axle wrap" effect. I would think a panhard bar or something similar would be needed. Have you ever put a front or rear end in and had to move by hand one of the spings a bit to get the pin in the hole in the perch? With the setup as pictured the axle would move to the side and both springs would move. The only reason that they don't lay over or move in a normally mounted fashion is because they are flat on the spring perch, the pivot you have designed takes away this "flat" element. Does that make sense? I'm not good at putting thoughts into words.