If your rad opening in the rad surround is only 22", I think I would open it up to 26" or at least something bigger then 22". A 22" wide rad is probably borderline for a 512 if you get in slow or not moving traffic on a hot day. Being borderline when the fix is simple and cheap doesn't make much sense to me.

The rad surround can be opened up with sheet metal cutting tools, you don't have to replace it unless its rotted out. You are modifying the truck, modifying the rad surround is not a big deal. Open that surround up and allow some air to flow through a wider hole and a wider rad. Gene