Don't use any limited slip lube in your eaton and it will stick a little better, but will chatter around tight turns and require service sooner. If you don't have 15" wide tires I would stick with the 400# springs. The harder you shock a clutch type differential on the launch the better they work. The new eatons use a carbon clutch pack, vs. the metallic one I have in my '69. I do have Scheiffer springs which I believe are 600#, they're yellow in color, and yes they are old. I've only obliterated my 12bolt once, and I've been damn hard on it. Its gone 9.80s on 9" tires with n2o on the launch.