I'm in the process of buying lights for my 180 also. I was thinking about buying two 36" fixtures also but they wont fit exactly right because it has a double center brace with 3 sections of 24" and I'm guessing that's the problem you're running into.
I just purchased 2 Fluval Aquasky Color + 36". Pretty happy with this purchase for my 180 Gallon 6 footer. Now, I have Frontosas and brightness is not my priority. For my needs, it is bright enough and love that I can customize the colors. This is very similar to Current USA Satellite + model, in fact the remote is identical.