I believe that all 67-72 GMC trucks used the 67-68 Chevy style radiator support with 4 headlight buckets instead of 2. So you can buy a 67-68 style Chevy support and cut the Chevy light buckets off, and tack weld the 4 GMC light buckets on it. GMC light buckets are smaller than Chevy's
There are no GMC radiator supports that I know of being produced in the aftermarket. My radiator support in my 1972 GMC was shot underneatch the battery box. So I got creative and cut out all the rot and fabbed up a new lower piece and welded it on the original. Not perfect, but after everything is all painted it looks fine. Plus, no one ever sees it as it's under the battery.
Gary