I have both a 3100 & 3600, 55 1st series. Makes sense that your 3/4 required a 3/4 thick spacer as my 17 X 7.50 wheel and tire assembly weighs 100 pounds. I'm going with a slightly thinner space for my half ton and will try a 1/2 inch and one a bit thicker. The spec book shows a heavier spring rate on the lh side, probably to keep the dpwnward movement under control on the sidemount side. The 3/4 ton does not identify a greater spring force, probably because the springs are stiffer.
