A couple of points were already mentioned (glass height, bracing). There's two more that I thought you should consider. Both can be worked through, but you need to keep them in mind:
1) Your bottom piece isn't 21" high like the front and side panels. It's 48" x 13" approx. It's going to look a bit odd with one side having two panels the same height, and the second side having one panel that's narrower.
2) The bottom piece of glass is typically tempered glass in 55g aquariums. This means that it'll be stronger than the other pieces and probably able to withstand the water pressure up to a greater depth, but if it fails it's going to fail catastrophically. Be particularly careful near the edges. If you hit the edge of tempered glass just right, even with very light force, the entire piece will break apart.