Maybe with a rimless tank it would be a good idea to go thicker glass but I am unsure if it would need to be thicker. I have seen a 120 gallon 48"x24"x24" rimless.
Will there be extra bracing at the bottom? Some manufacturers will add a second layer of glass around the bottom seam for more surface area. At only 18" tall I would think you could make it work, but 5' is getting pretty long. I would try getting in contact with a couple of reputable tank manufacturers and see what they say on the matter.