Hi Stan,
Just read thru the posts and not sure if I can help, but will toss my 2 cents in. I have built an 87 300zx into an ITS car and am running 450 up front and 350 rear. These rates were suggested by a car builder out of atlanta and seem to give the car a real good feel compared to stock. My car is a pig, over 3000 lbs, and I don't think the 510 is that heavy, but we have a similar rear suspention. By lowering the car you create that nasty negative camber in the rear. The bad part is under breaking, that negative camber lifts up and causes a shift to almost positive camber, under stock conditions. You can imagine what that poor tire patch is going thru. Rule #1 in a Z has always been "finish your breaking before you turn" because of that. I find with 450# up front(stock is around 160-180#) that the front end doesn't dip as much under braking which prevents the rear from picking up. Oh, here is a link for slotting that crossmemeber before I forget...
http://www.nexcent.net/datnet.org/link_0001.html

There are a few pages of people who modify the heck out of these cars. Hope this helps.