Why do you want it 40 inches high?
You won't be able to get in the tank to clean it or set it up. It will also be top heavy and potentially unstable.
Normal glass thickness for a 6ft long x 2 ft wide x 2 ft high tank is 10mm thick. That is if the tank has cross members and support strips on top. If it doesn't then you go to 12mm thick
If a tank is 6ft long x 2ft wide x 30 inches high, the glass is 13mm thick if it has support strips and cross members, and 15mm thick if it doesn't.
I haven't built a tank that is 40 inches high but I am guessing it would be at least 20mm thick glass and that is going to weigh heaps. It will also cost a lot.
Most people that want really tall tanks will go with Perspex/ acrylic rather than glass, and they get a specialist tank maker to build it. If you use glass it will need several weeks for the silicon to dry.