radiators.com has good prices too-use Mopar red, Zerex G-05, or Motorcraft Premium Gold, they're all interchangeable, many times a sympathetic Ford dealer has the best price. Don't tell them it's a Chrysler...
If you can afford it, OEM Mopar radiator, unless Hella Behr or Nissens sells a radiator. The Chinese or Taiwanese ones vary in quality. TYC/Koyorad and Spectra Premium seems like the better of those.
Stick to G-05. The Mopar HOAT is orange in color, while Motorcraft Premium Gold is yellowish. The gold bottle Zerex is clear, so it's not a bad idea to use a coolant tester or refracto/hydrometer to verify concentration. Mix it with distilled water. CA does have hard water unless you live in SF/LA/Tahoe or the EBMUD district.
Thank you everyone for the help, and it has been a big help.. I did get a MOPAR replacement radiator. Found a good price and the part I wanted. My 300C has the 5.7l in it, but found the radiator for the 6.1l drops right in and is 0.4" wider! I did also go with Zerex G05 HOAT coolant. So far so good!
You did good with the Mopar radiator. I needed one in a pinch for my Jeep so I bought one at AAP. Unlike the OE part, this one is only a single row.
Not so much an issue for me in the Midwest, but I can see it really making a difference in the desert. I also added a mechanical fan on top of the factory e fan.