so, are you building a stand for a 55 gallon tank? im confused. also, is this a glass or acrylic tank?
anyway, if you are building one for a 55 then listen up. i made a 2x4 frame to support the entire base of the tank, and a 2x4 frame to sit on the floor. my tank is glass, but this stand should work for an acrylic also (as long as you make a flat top for the complete tank to sit on). at the corners, i used 2 2x4's standing on end, and i screwed them together to form an "L". these were the legs that went between the top 2x4 frame and the bottom 2x4 frame. i used some heavy duty corner bracket (NOT those dinky little L brackets) to secure the legs to the 2x4 frames. I also added a vertical 2x4 in the front to break up the front face and provide an opening for my doors. if you use 2x4s you shouldnt need a brace in the front or back... but like i said, i added one for aesthetics.
so, if you build the basic box frame for the stand, then you can use whatever you want to increase the outside dimensions. just remember that the parts that hold up the tank need to be sturdy. the center vertical braces are not needed for a 55 gallon tank stand made from 2x4's.
i hope this makes sense... if you have questions post a sketch of what you want to build.
edit: i used Simpson Strong Tie part RTA2Z for my corner bracing... http://www.strongtie.com/products/connectors/RTC.html