you're right, that is too high for MM coilovers. Did you install them correctly, I.E. did you weld the ring for the threaded sleeve in the right location to allow for enough adjustment in both directions?
is your car sittin on the bumpstops? (it really shouldn't be if it is a 280z at that height) my 280z with eibach "lowering" springs puts me down a little lower than the ride height in the pictures with about 2-3 inches of stroke left on the shock absorbers. Oh yeah, I run the same size tires too, so it is pretty easy to compare.
heres a pic for comparison... kind of a bad pic, if you want more "reference" pics i can shoot some tomorrow...