You don't need to. I didn't, my car had a 22" radiator and core support and now has the Champion 26" radiator, and I haven't had any issues with cooling. The radiator support opening only controls air flow from the front, with a proper fan shroud the fan will still pull air over the entire surface. And it's not like the edges of the core aren't exposed to the air, they aren't in contact with the radiator support. There's still an air gap around that part of the radiator core. It just means the very outer edges don't get direct air flow from the front. I suppose it reduces the efficiency slightly, but at least for me it's not an issue at all.
Also, think about when the uncut radiator support has the biggest effect- it's when the car is moving, which is when you're least likely to have overheating problems. Keeping the car cool when it's not moving, or moving slowly (traffic) is usually the biggest problem. And in those situations it's the fan that's moving all the air. So, if the fan shroud on the back covers the whole radiator, the fan will still pull air across the entire core. The radiator shroud sitting out in front of the edges of the core isn't going to stop the fan from pulling air through the radiator.
