Radiator questions

[SharkPilot]

It's never as easy as you think it will be.

Now that it's summer and getting hotter, my car was running a lot warmer in traffic. So I pulled out the radiator to clean out the bugs and assorted debris. The core is really plugged with dirt and flushing it with water and blowing through with air didn't make a huge difference. So I chose to replace the whole radiator. Since I work at a GM dealer ordering a new part is pretty easy to do.
Now the hard part. Today I picked up the radiator (pt. #52473260) and when I compared it to the original one it looks perfect, except for the thickness of the core. The old one is about 1 1/4" and the new is only 1". The GM catalog shows it to be 1 5/16" and when our parts guy contacted GM techinical they also stated that it should be 1 5/16". The GM warehouse where we got this one measured another unit in their stock and its core is also 1".
Nobody can explain what's going on here. I'm wondering if anyone else has encountered this problem and if so what did you do? I'm rather wary of giving up 20% of my cooling capacity to a thinner core. Should I go with an aftermarket unit? Those are $$$$ but if that's what it takes...

Thanks for any help, SharkPilot.