Back when I was in the military
I worked as a truck mechanic,a bulletin came out that we were to inspect every rivet on every truck for sings of looseness.The inspection was a visual one,if you saw light rust residue around the head of the rivet you knew that it was loose enough to be chaffing against the chassis or the crossmember.We replaced all of these with grade 8 hardware torqued to specs.I'm just saying this for you to consider,if it was me and I had to replace a crossmember I would go with the grade 8 bolts,if it was for a restoration the I would use the rivets.When you look at it those rivets are keeping the integrity of your frame together,there's a lot riding on them.HF