Well, the opening is just that... about 22". So, if you want to utilize all 26" of your radiator you would need to enlarge the opening to each corresponding way to accomidate the 26" rad. You're not done there yet though... you still have to drill for the brackets of the 26", then find upper and lower hoses that are long enough to reach to the inlet and outlet on the rad.
Now, it depends on which A body you are referring to when you say 340 and 383 cars, as a 1973 and later Duster, Dart, or Scamp with a 318 and A/C could have come with a 26" radiator, no??? Correct me if I'm wrong Duster guys!! So, without identifying the body style, you are just shooting in the dark with your question!!! Mor info please!!! Geof