Because it has more volume cooling capacity and surface area for heat dissipation, regardless of the opening....thus more cooling power. For mounting the 26" radiator, use the two factory pre-drilled holes on the driverside, and like 69a100 mentioned, drill the holes for the passenger side. There is two stamped indents on the passenger radiator cowl wall that are blank. They fall perfectly inline with the two stamped indents w/drilled holes for a 22" mount, just further out. That's where you'll need to drill out, unless you go with a custom bracket like 65Hemi.
As far as the shroud, on a 22" it's opening center is offset to accommodate centering the fan. From the factory, they were that way. 26" ends up being centered. He'll have no issue with a factory style shroud.