I just went through this process...I stuffed as much as I could under my 55gallon stand.
The stand is your basic pine stand from the LFS that measures 48x12, however the interior dimensions are a bit smaller. I only had 10 3/8ths inches interior width. Extremely frustrating seeing as my proposed 10g sump (glass aquarium) was 10 5/8ths...so I had to cut the rear center support beam and re-mount it so it fit entirely on the outside of the stand. Fairly simple carpentry, but just have to make sure you get it right because you are going to be putting roughly 650+ pounds of water, rock, and sand on top of that.
Heres the link to my build thread... you can also see where I removed part of the back right support beam to fit the fuge & sump lengthwise.
http://www.reefcentral.com/forums/showthread.php?t=2069247