adjust your spG by adding salt water not fresh as top off for a day or two that way it will move up slowly and it shouldn't be an issue.
yea i think you have way too much rock that "rule" doesn't account for the fact that most corals come attached to a small rock and need room or the density of the rock - its really a useless "rule". sorry to say but i would remove about half of it
i know that sounds drastic but you'll see what i mean as you start to get corals in there - less is more IMO........plus it is very nice to have some free sand space up front to put those corals like to be in the sand.
so what to do with the extra????????? i don't know, some could fit in the back areas like ed suggested, but not very much and i doubt they will take it back but you can try..... its really nice looking rock! you may have to just set up a bucket of salt water to keep it in should you want it in the future...... sorry don't have a suggestion for you i know thats a lot of money your out there