The 26" wide radiator is offset. Take a look at the radiator shroud picture and you will get an idea on just how much. Actually, its the engine tat is offset. But you will get the picture. To answer your question on the opening. Yes there is a panel that is sold that replaces the passenger side yoke with a narrower one to widen the opening. However, I am pretty sure the driver side yoke is also different. I can't find the pictures right this moment, but I believe there is a 1/2" difference of the opening and the captured nuts that hold the radiator are over almost as far. I think there is enough slop in the radiator and shroud mounting that this doesn't matter much. Which is why the one side is probably offered.
Now a 26" wide radiator will help as it gives more surface area, but to address your cooling issue, there could be some other avenues to go down before getting the spot weld cutter out. First is try to find a radiator shop in your area. I had a 4 row core installed in my 22" radiator for my 68 Road Runner. I also increased the fine count from 12 fins per inch to 14. You will definitely need the 7 blade fan and a shroud as this will add some pressure drop across the core. The shop did widen the lower tank as it was only for a 2 row core. This kept my stout 440 cool with no issues.
