Hi Andrew,
The rock we have in stock is now cured, but many people went ahead and bought the rock uncured and semi-cured, so not much left.
If you don'd mind waiting longer, we have a fresh 1000 pounds coming in saturday. We could take an order when we get back from vacation and set your pieces aside until they get cured, that way you get top picks.
On the density, 80-90 should be plenty for a 55gallon, that should pack it nice and full. Density varies from batch to batch and piece to piece, but usually it's pretty light with lots of holes.
jason