Rampardos gets Head Smash but doesn't have Rock Head, even though it's the one Pokemon who should logically have it over anything else. I mean, Aggron gets to have a recoil-less STAB Head Smash and I don't think anyone would consider it overpowered. And even with Rampardos's much higher attack power, it's craptacular defenses mean that it still gets demolished by priority moves, so I feel like this addition would hardly break it. Maybe give it one more round of action. Two at most.
And on the subject on Gen IV fossils, Bastiodon needs Solid Rock like yesterday. Camerupt gets it and it's not even a rock type!