I just got a 26" radiator for my dart from here....
http://www.speedcooling.com/26-Small-Block-Mopar-Radiator.html

I was very surprised at the quality for the price. It will be a while before I actually run it but from the looks it should work quite nicely.
I swapped in some 26" radiator supports on my car. I believe you can use a 26" with the smaller opening but some of the core will be covered. Not sure if you would really benefit from the larger radiator without changing the supports.