I had a 54 gallon corner tank for a while, and it was a pain to landscape. I learned that with a corner tank, you kind of have to throw the usual rules of aquascaping out the window. For example, even though it's usually recommended not to have the main focal point in the center, in a corner tank it often looks best that way. Embrace the triangular shape. So, I would suggest putting the big rock more towards the middle - maybe not exactly in the middle, make it look like it's just naturall there. I would also suggest using it in a more upright position. I think it'll look better and take up less substrate space. Have you tried it so that it's sticking dramatically upward, more straight up than your first placement? Then position the other rocks around it so that they look like they're part of the same bunch of rocks that just naturally eroded off the big one. That's what I'd try anyway.