Wow! Easy here boys, it ain't no Railroad bridge. The strength is a non issue. You are talking thousands of lbs strength (uh like 3/8= over 6000 lbs!) X 4 or more bolts to hold a cross member that might carry 200 lbs. A grade 5 will be plenty. A 3/8 bolt has a 3/8 unthreaded shank. Use that like a shoulder bolt with a lock washer and nut. It would be nice to have the unthreaded shank go all the way thru all the pieces. You don't need locking compound for that kind of vibration levels. The real issue is what will the Gendarmes want? They know nothing about rivet/bolt strength. So you may want to use a Stovebolt/Carriage bolt head that looks like a rivet. All big structures now, bridges, buildings, etc. are all bolted. No rivets anymore. Except maybe a few in autos where they want to hold a panel in place while boding cures. They also use self tapping screws, MIG & spot welding, and a lot of bolts.
Of course the Aerospace industry still uses aluminum rivets along with spot welding.
