2.5 feet of glass needs at least 1/2" of thickness. I'd go 3/4 or don't do it. Seriously its not just for fish that hit it. Its so there isn't shards of glass rocketing across the room. If your lucky it would just crack and leak.
I recently had a quote for an all glass tank of 9'x4.5'x 32" high and they were going with 15mm so i guess they know what they were talking about so i wouldnt go any thinner than 1\2" on your build.
i guess im just going to have to keep shopping around. the biggest ne glass shops carry around here is 3/8" - well thats what they have in stock and can order other. might just have to go with a little smaller tank. i just want to make sure that i have enough room to get 2 payara (hopefully breeding pair).