you can go about 3" lower on 280ZX struts without having to worry about bottoming out on the strut itself. the hard part would be finding decent shocks that fit in stock length housings.
Some guys use MR2 Tokico shocks, although I don't really know how much shorter those are than 280ZX inserts.
there's a really nice thread or two about this on ratsun.net, where alot of 510 guys use 280ZX struts, section them, then go real low with their dimes
for springs and whatnot, either get some cheapo coilover springs and sleeves for like $80 which should be in the area of like 400-500lb/in springs, or you could get Ground Control coilovers and have them be in the area of 250-300lb/in springs.
it all depends on the ride quality you're looking for.