I know when I had my aftershock, I drilled out the top pivot ball and tower for 4mm bolts. I think they used plastic/nylon pivot ball stand offs. On the bottom though, they were captured, so the 3mm bolt worked ok.
You might want to try RC Raven's dual rate springs vs the strange setup you have with the springs around the shaft. I really liked the dual rates on my aftershock. Ran a little heavier oil and it could take a lot of air without smacking the ground too hard, but still was soft enough to not bounce around on rough terrain.