Less is more IMO. I like to do arches with blank spots then an island off to the side. That said though, rectangles can be a challenge to aquascape. It's the reason I fell hard for cubes some time ago - more real estate to play with. The 75 is a nice size, much better than a 55. You just have to get creative and like someone else said, make sure you don't clump them up against the glass where cleaning will become a challenge.
If it were my 75 (which I ran one for some time), I would take some of the rock from the left side (liking the arch you have going, leave it would be my choice) and make part of the right side shoot up closer to the water line, then either one or 2 large pieces on the left side kind of out by themselves (island-like).