I agree with Jerry that if you can't rivet an attachment, a tight fitting shoulder bolt would be next best. However, I disagree that a standard bolt tightened to the proper torque would loosen up and make the holes eggshaped. The clamping force of the three or four bolts for a shock mount will keep it in place for the life of the truck.
The shoulder on a shoulder bolt will need to be just shorter than the combined thickness of the frame and the attachment so that the nut, when screwed on will clamp the two together tightly. I think the complication of getting the right shoulder length and reaming the hole for a tight fit makes their use out of the range of the typical DIY guy. Of course Jerry has the skill and equipment to use them.
