I ran a big truck radiator in my 48 3100 that the radiator shop put a bigger upper neck on for years until the core gave out. Old cores are old cores and finding one that is in good enough to use is a challenge now that they are over 65 years old.
OOPS, THIS ISN'T GOING TO WORK IN A GMC WITH A GMC CORE SUPPORT. You need a radiator with the filler neck curving into the back side.
Brice Thomas will custom make a US made radiator but they aren't cheap
https://cgj.com/
Looking at the champion that Chopped53 posted that may be your best option. I did put an inquire into Engineered cooling products to see if they can do one for a GMC.
I have one for my truck from Engineered cooling products
This one
http://www.speedcooling.com/1948-195...-Radiator.html
I had it out of the box yesterday to take some photos to show someone on another board what the companies's work looks like. Painted black it would look like a stock radiator outside of the tig welding if that was the look you wanted rather than the aluminum.