i recreated my whole besement(with the bulkheads pipes and proper angles) to scale and the furniture in a couple of houres to see where i can fit a huge tank and how big, here is a pic with the existing inwall and the "proposed" tank, i moved the furniture out of the way to take the picture...
the tank i came up with is 320gallon and if i get rid of the window(which i dont like anyway) I can make it a 500-600 gallon. and it uses the parts from my 120 to make it.
I love it and recommend it highly...