Kevin,
Since the Ibeam is 8", are you talking about ripping a 2x4 and toenailing it onto the edge of the 2x6 girt? This approach would limit horizontal deflection, but I am worried about horizontal deflection. Maybe I am not seeing something, why don't you like the double 2x4 girts. I even considered putting the insulation between the metal skin and a 2x6 girt and paneling the internal wall, but this is more expensive than the double 2x4 idea.
I plan on cutting scrap lumber and nailing them between the girts, thus helping with the vertical deflection.
interesting conversation...keep it coming.
Mike