68 Barracuda 440 getting hot in traffic

Here are some measurements and pictures of the 22" radiator support with a 26" radiator behind it. Let me first say that if this was even an issue, the loss of cooling efficiency would be at speed. That's when the airflow through the radiator support would have the largest effect. At idle, the fan is moving all the air. If the fan shroud covers the entire radiator, and the core is not directly in contact with something in front of the radiator, then the fan will still pull air across the entire core.

So, here's the opening on a 22" radiator support. Mine actually measures 22 3/4", so right off the bat there's only about 1 5/8" of core on each side of the opening.
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This is what you're "losing", a 26" radiator support would be open back to the red dotted line (in line with the lower reinforcement)
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But you can see from the top that while the radiator support opening is smaller than the core, the core is still surrounded by air. The support does not come in contact with the core, so air still circulates through it
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A picture from the side showing the air gap. There's a good 1/2" or more. You can even see some little piece of leaf back there in the corner, wouldn't have blown back in there if there wasn't airflow right?
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Bottom line is you still get more cooling out of a 26" wide core assuming everything else is equal. Yes, at speed you will have better airflow across the middle 22 3/4" than the 1 5/8" down each side, you lose some air speed on the edges and therefore a little efficiency. But if your car isn't cooling at speed that little bit on the outside isn't going to save you, something else is wrong. At idle the fan will pull air across that entire core as long as the fan shroud covers the whole radiator. The fan is pulling air at a much slower speed, and a 1/2" air gap is plenty to pull air around that lip and through the outer edges of the core. The back side of the radiator isn't obstructed at all compared to a smaller 22" core. And even if the outer tubes are slightly less efficient, they're basically bonus compared to a 22". It's all extra cooling that a 22" can't provide because it's not there. Not to mention the increased volume of coolant from the larger tanks and additional tubes.

Regardless, the OP has a 26" radiator already. Buying a 22" would still be a downgrade, and cutting the radiator support isn't necessary at all if the car is cooling at speed.