Did you decide on what steel to use yet? If not, it seems like a lot of people go with 2” 11 gauge or 1/8” wall tubing for larger tanks.
I am planning a stand for my 65 gallon right now (48x18”) and will be using 1.5” 11 gauge for the bottom and sides, with 1.5x3” 11 gauge all the way around the top so I don’t have to use vertical center braces.