You can use compression couplings. The borgs even sell a kit that has 2 3/4" compression fittings, 2 nipples for the tank, the teflon tape and two flex pipes that are about 18" long each.
You might want to check this out. My local ACE Hardware dealer says plumbers use it and like it.
http://www.justforcopper.com/
and http://www.youtube.com/watch?v=1HX6ahDqoY4