Lots of good information being shared, when I rebuilt the frame on my Pat 12 C60L I had to replace all most all the rivets as I had to replace the inner frame completely and replace a number of sections of cross members that had rusted out sections. Looked at riveting but that seemed to be out side of my technical ability so I went the bolt route.
As suggested I line bored each hole so that untreaded section of grade 8 bolts were a drive fit. Then drove the bolts home and torqued the bolts to 75% of max for each size. Line boring all the layers at each joint with the different plates clamped made for tight fit. I used a small drill press clamped to the frame to hold it square while drilling. see
http://www.canadianmilitarypattern.com/Tools.html for picture of little rig. The drill press also made drilling the holes easy.
One point I would add (obvious but worth saying) make sure that you have the frame perfectly level and square. Though the pictures of drilling the chassis show the chassis on the wheels this was only after making sure that it was actually flat otherwise I would have blocked it up of the floor. In the pictures you will note little numbers next to each hole in the frame these are the bolt sizes used in each case.
As to correctness as a restoration, yes I agree rivets would be better, but in reading old truck manuals replacing broken rivets with bolts was common practice. Some of the repair descriptions I have read recommend redrilling the holes to a larger size for a tight fit of the bolts all layers.
In driving out the rivets on my frame I did notice that the hole in one piece of steel would be larger than the other so that the rivet was actually expanded in the hole making for an even tighter fit than just mushrooming the head.
Cheers Phil