http://www.ebay.com/itm/161697190569...%3AMEBIDX%3AIT
http://www.ebay.com/itm/150980376848...%3AMEBIDX%3AIT
Sometimes these boiler plate compatibility links used by Ebay sellers don't have GMC figured out. One of these has GMC in their heading. They both say yes for 1961 Chevy C10. Ask for the size if you are not comfortable. They should work fine. Also: use the search function here and you will find info on what others did.
Here is one from the search. This person says the first link I gave worked for his 1961. CC6062. $204.85 Free Ship.
"I looked at getting my original rebuilt but they wanted over $450. My old radiator constantly ran at 210-225 degrees and that was in the cooler months in Phoenix. Did a lot of research and found the Champion radiators had a very good all aluminum 3-row, great price, bolted right in, had to use a different flex lower hose. Got it from Ebay from a dealer out So.Calif for $200 with freight. It has yet to go pass 190 degrees idling, about 180-185 driving and that was in 114 degree temps this summer. (Champion #CC6062) they have many different fitments.
Rick Winfrey Phoenix, Az 1961 GMC LB Stepside Sent via BlackBerry by AT&T"