If it was my tank I would get a piece of glass about 6 to 8" wide, enough to cover the crack and silicone it in . I would not replace the bottom frame but would set it on a layer of styro. The local Big Al's has a shark tank that is 4' x 8' x 3' tall that has a cracked bottom that was repaired with a patch. the tank holds in excess of 700gallons. Somewhere on the net is a pictorial story about the building of a 16 ' long tank that has 3 pieces for the bottom.
I have been waiting for someone to post something saying that they've seen successful patches in large tanks. I really appreciate the input, I think I'm going to give the patch a shot and just let the tank sit with water for a few weeks before I put any fish back in. Gonna get 1/2" glass, I figure I'll get a piece about a foot wide to cover the crack plus some.
I just bought a 250 gallon tank that was cracked along the bottom. what i did was get a custom peice of tempered glass allready drilled where the overflow and return were and cyliconed it right on top of the cracked glass. i also put a bead of cylicone along the crack.only hard part scraping the old cylicone off along the bottom that sucked because it was such a big tank.
Nice save! Any pics of the tank?