There are a couple things that are required, IMHO, to make a bolt-on work correctly. Note that most (but not all) that are not welded on by the OE are riveted rather than bolted.
Rivets work slightly different than bolts. The action of swaging the end also causes the rivet body to expand and completely fill the hole. That's not something that a bolt can do. The best that a bolt can do is to precisely fit the hole. To do that the hole must be reamed to size and not just drilled.
Then there is the properly sized and installed requirement. To me that means using a bolt with a shoulder that is long enough to fit most of the way through the washer under the nut, using a prevailing torque, all metal lock-nut and two hardened, heavy flat washers positioned with their rounded faces against the frame and bracket. Then each bolt should be torqued to it's correct torque value. If I'm feeling excessively anal-retentive about the install I'll also put Lock-tite 271 or 620 on the bolt threads.
If the frame is a box section frame then it will also need to be sleeved for the bolts to pass completely through the frame. Which can be a can of worms by itself. The simplest way that I've found to do this is to drill one side larger than the other. Unibits are a godsend for this work. The small hole is a clearance hole for the bolt used. The large hole is the OD of the sleeve, and I make the sleeve the width of the frame rail. Which means that it stands slightly proud at the large hole side, where I can weld it in place. I choose an OD that clears the flats of the nut, say roughly the OD of the socket used on that nut and ream the ID to the bolt size. In this case only one flat washer is needed.
For the new upper damper mounts on the front of my FJ60 I am welding them on after first welding on a similar thickness doubler plate that has appropriately placed rosette holes in it. I am putting on the doublers because in using the F-250 towers there is no way to avoid a vertical, straight weld bead on the side of the frame rail. So I'm moving that bead to the doubler and have shaped the doubler to not have any straight up/down weld beads.
I used to swerve around my hallucinations, now I drive right through them.