I bought mine off of Ebay as well. I had issues with the 5/8" radiator, that the car had when I got it. In temps above 85 down here and the 5/8" wasn't enough to keep the car cool at a cruise I fought it for a while with making sure the ducting was in place, making sure that the air couldn't escape through the condenser everything but when I would get to the freeway the car would climb and climb from 190 up to 217 and the fans would have to run. I bought a 1" off Ebay that turned out to be a 7/8" and went back and forth a few times about their description being a 1" and the actual thickness was .891" which would be an internal 7/8" after that switch I was able to cruise in 100+ temps and still remain at 190* so the radiator makes a difference. at least down here where the temps are over 90 for most of the year. I am still running the same 7/8" core rad even though it's bent and still maintaining 186-188 at cruise. I bought a new radiator to replace it on Ebay and bought from a different vendor and ended up getting a 1" core. They are all the same price so get the largest one you can. I wouldn't recommend going with the other cars radiator since it requires different mounting than the 04-05 radiators since it seems you have an 05' I have an 05' GTP as well.
Jeff