It will be very difficult to get the bracket "UP" tight AND "IN" tight unless all holes are line drilled while bracket is held, up and in, tight. If holes are ovaled out in one direction and not the other, that will bolt up nicely and be OK. Tolerances of the frame and the bracket hole locations may cause gaps. As you know you are bolting in two planes. The slightly larger holes may be to your advantage. After bolting, it won't move. Torque per size chart. There are only two choices: line drilling things dead nuts or provide clearance holes to accommodate location tolerance. Riveting "eats" some tolerance. Also the frame is not a perfect 90 deg and neither is the forged bracket. Some Gaposis is OK. Not trying to make this sound like rocket science, it's not, just a bolting discussion. Agree with Joe about shank. The bolt will be stronger if the threads are not carrying the load.
Seeing things riveted makes us think they have to be bolted like riveted. Not true in this case. Can't move up or down because it's clamped, can't move in and out because it's clamped. It could only move forward and backward, but all four holes would have to be off in the same axis on the correct side of the bolt because it could only move straight back. As Carl says something really wrong happened. Do I see a slightly different head on those rivets? Again, with Carl, I gotta say four or more factory rivets don't come loose. Could be a bad PO rivet fix. Use bolts and lock washers this time. Try to find USA made lock washers. Grade 5 bolts will be plenty strong. As in: 4+ tons per pair for a 5/16 bolt.
