I just left the opening at 22" on mine. I've never had an issue with the temperature climbing while out on the road, which is where that radiator support restriction might show up. And the Contour electric fan set up I run has a shroud that covers the entire 26" core on the back, so when the fans are running they pull air across the whole core. I'm sure it's a little bit less efficient to leave the opening at 22" with a 26" core, but it's a pretty small difference if there is one.
