i've built many stands for large tanks.
basically if you use doubled up 2x4s to make 4x4 vertical posts ever 1.5-2 feet. it'll hold it no problem.
for the top use 2x4s on edge and brace them from the front to the back of the stand
heres a picture of a stand i build a couple months ago. the 4 corners of the stand had solid pillers that were 4x8 inches solid wood
