Well, I built the stand for my 125 and the way I did it was to frame it taking into considerations the sump. I made the end panels with large doors and can fit a 55aga through with about 1/2'' to spare. If I take the whole side including the door off the frame there is slightly more room.
Take a look at your stand and measure out the opening you need. Then figure what braces will have to go in the corners for support. You should have enough room if the 2x4s run wide tward the back and front. My preferance is to slide the sump into the stand from the side.
One thing to remember, there are many things that could be used as a sump, from rubbermaid tubs to acrylic to just a skinny tank. I think a 55 is a good size to shoot for.