No, it wont fit. I have a 55 under my 180g and the only way to fit it in was remove the top middle support and place the sump through the top.
This does work, but makes sump maintenance an obvious pain in the ass. I wanted a large sump so I thought it was worth it.
A 55 is 12x48x21. It might fit if you put in before the tank is in place.
I am using a 55 as the sump for my 220 and I am thinking that I will have to put it in before I put the 220 on top. I am going to modify my stand a little bit to raise it about 8" and beef it up so I should have some decent clearance over the top of it.