I have been struggling with overheating issues too. Tried radiators, fans, etc... I have finally figure out it was my heads. There is most likely na warp or crack that is letting compression gases into the coolant system. These gases are super hot and are overwhelming the systems ability to cool effectively. I doubt you have a dud radiator. I suggest you look at other possible issues.
If a leak is suspected, or your can't cure your overheating, there are a few ways to check for blown intake/head gaskets or cracks.
1) Watch your liquids. If oil/carbon is mixing with your coolant or coolant is mixing with your oil, that's an obvious give away. Rent a radiator pressure tester from Autozone/Napa/etc; this replaces your radiator cap, you pump it up to induce pressure to 13lbs, and then you let it sit for a while and see if the pressure decreases or your can hear it hissing out through a blown seal or block/head crack. Edit - you can also rent coolant dye which'll change color at the presence of combustion gasses in your coolant, which is another way to check for leaks or cracks.
2) Check your spark plugs; if one or two are really clean and the rest appear normal, this could signify that coolant is getting into those cylinders and washing the spark plugs.
3) With your engine cool, remove your radiator cap and start the car. Without driving the car, slowly warm up the engine idling until it gets to operating temp and the thermostat opens; with the radiator cap off you should be able to see the coolant flowing across and as you SLOWLY increase RPM you should see the flow rate increase to signify your thermostat and water pump are functioning.
4) Running a cylinder compression check on all 8 cylinders can also tell you if you have a crack, although a low reading can also indicate valve or ring leaks, not necessarily a crack or blown head gasket. But if you have 1-2 cylinders with much lower compression than the rest, that'll be a red flag.
5) Does your lower radiator hose have the spring installed?
6) are the tabs from the head gaskets sticking out the front signifying you installed the head gaskets in the proper orientation?
I also installed an upper radiator hose filter as my past radiator had plugged up from engine sediment, and sure enough, even with a new engine which was hot-tanked and cleaned well, it caught a bunch of large chunks of metal/gaskets/rubber/etc for the first few hundred miles.