Good question! I have the 22" opening and would rather not cut it.
I run a Champion CC374 in my Duster. That's a 26" 3 core that is listed for B/E bodies. The Duster had a 22" radiator in it originally and still has the 22" opening in the shroud. All I did to mount the radiator was tack weld a couple of nuts onto the front of radiator support like the factory did for the 22" radiator.
There's no problem putting a 26" radiator in front of the 22" shroud opening. In fact, it will still cool better like that than a 22" radiator would. You still have more surface area, and that surface area is still exposed to air that's going to be cooler than the radiator. Yes, the radiator would probably be a little more efficient if you opened up the radiator shroud so that the entire core was exposed, but we're not talking about much.
I run my Champion CC374 with a Ford Contour electric fan set up. I swapped to that because the car ran so COLD with the mechanical fan that it would take forever to just warm up to operating temperature. How's that for a problem to have? Anyway, everything works great with the Contour electric fan set up, they rarely come on when I'm actually moving, usually just at stop lights and things. Even when its 100*F or more they don't run continuously.
That's on a '68 340 with ported iron heads, 9.8:1 compression, bored .060" over, and making in the neighborhood of 420-430 hp.
And, there are quite a few threads on this already. A few folks have had some quality control issues with the Champions, and pretty much everyone has to drill new mounting holes because the "factory" ones are in the wrong spot for these cars. But, for a radiator that you can often get for around $200 depending on the model, that's not too bad.
