Steve, the reason for going slightly over 48" and 24" is because a 120 is slightly larger once you consider the black frame that goes around the glass at the top and bottom. If the stand is exactly 48x24 it will be slightly undersized. (Unless he buys an acrylic tank)
Ahhh, that's why I stick to nanos. Next I'm going to do a 10 gallon seahorse/pipefish tank on a file cabinet because what better idea is there than having 10 gallons of water over your really important papers.