Can't ID it, but the extra removable plug on the right side would indicate its an emission era mid- late 70s radiator, that removable plug would be for a temp sensor for the dist vacuum advance or something else.
Or it could be an industrial use radiator, lots of slant 6s were used in industry for fork trucks and such. Can't tell how thick the core is, most industrial applications would be thick 3 core radiators.