If your 650 is bucketing so badly I would be seriously checking the fluid in the forks!
650's don't duck and dive like that unless the springs are knackered or the wrong fluid or levels are way down.
Also remember there may be some carb issues if you lower the rear too much and the bowls may tilt back and upset the floats....probably not but hey if the bike starts to cough when you ride it could be something to check.
Good call on the 400 shocks, the lowering blocks really upset the rake and cornering of these bikes, the rear shocks end up set too far back to travel correctly and you might as well have a rigid frame.