Bmilleraz,
From my experience I would go with 1.5" for a salt fish system, and 1.25" for a Fresh system. At almost 8' you would get more than exceptable bowing. If the joints are tight, it will not pop, just belly out too much for me.
How do you plan getting to the bottom at only 20" wide? If the tank was from me the I.D. would be 17" and I know my arms would only get half way down!
Best of luck,
Jason