Just from looking at the cover of #27 on the GCD, i would guess that very little would be acceptable here, since at that point, Popular was still primarily reprinting comic strips. Most of the licensed Dell titles have already been declared (rightfully) off-limits already, and I would guess that these early reprint issues would be the same. The later issues, like #66, seem to have moved to new material, not reprinted, including the Professor Supermind and Son material. if that material was not renewed, it would be PD.
but I agree, we need to wait on one of the mods on this one to be sure.