I think, with the question of the condition of some of the existing holes, I would have a machine shop drill, or re-size the holes and install the studs. It would suck to go through the effort to install them yourself and discover one of the holes isn't centered because the threads were wore on one side of the hole and not the other, and the hole was drilled off center and now the new stud is off center.

Another thought might be to hane the machine shop relocate the new studs to the current locations of the rivets. The would solve the stud hole and the rivet issue at the same time. Gene