I used a 6 MM drill, the holes are 1/4" I believe. Then used 1/4" nuts and bolts, with dab of light Loc Tite on each bolt. Hammerite paint works well on rust, then a coat of wax to seep into the cracks. The door hole undercoating (odorless) works well, wax base.
The 6 MM drill cuts the heads first so most times the rivet without the head just pops out the back. The 6 MM is just a fraction larger (actually almost exactly) the size of the hole. I may have used a drill slightly larger than absolutly necessary, but it sure made getting the rivets out a lot easier. I also drilled at a low speed, with a sharp bit.