While normal grade 8 bolts will probably work they are not the best for the job at hand.
If you must use bolts, be sure they fit snugly in the holes. You will usually need to go one size larger than the rivet you removed. The reason is that rivet holes are punched with a slight;y sloppy fit, then the rivet while hot squishes out to fill the hole. Your bolts won't squish, so properly size the bolt holes for a snug fit.
I have used "frame bolts" and the matching nuts. They have a washer like head on both the bolt and nut, they are grade 8, and the nuts are the crimped Stover type lock nuts. I buy mine from a big truck parts place, they have larger ones, and order the small ones for me.
I personally prefer to use hot rivets for anything on a frame that does not need to be removed again. I reinstalled about two dozen missing, damaged, loose, or improperly installed 3/8" hot frame rivets on my 48 two ton. If you really want to do it it can be done. I much prefer rivets over bolts when a rivet was there to start with.
If your frame should be put together with bolts they would have used them to start with right?
Search for rivets on this site, we have had this discussion before.
Grigg
