ive got the same measurements on my 72 Demon and I used a 26" radiator the 1 1/8 that overlaps the sides is no big deal works great sit in traffic all day long and doesn't overheat
My '74 Duster came with a 22" radiator and it has a 22 1/2" opening. Just because it had a 26" radiator doesn't mean it got a 26" opening in the shroud.