With 5/8" and 3/4 bolt holes I usually use flange bolts. However, for a 3/8" hole I have been using crossmember bolts for over 40 years.
https://www.imperialsupplies.com/item/0009790This has been the standard fix for semi trailer cross member to bottom rail repairs. Most trailer manufacturers use buck rivets or Huck bolts to attach the floor cross members to the bottom rail. Salted roads cause things to swell and rivets and Huck bolts break. I have a dozen dry vans in drop and hook service. I have used hundreds of these bolts and have never had one to break and they are simple to install. If the hole is a little too big or out of round it doesn't make any difference. Our floors stand up to a beating with forklifts with solid tires. A pickup frame will never take this kind of beating. I buy them by the box of 100. If you are going steel to steel you don't need the nylon washer. That is for steel to aluminum to prevent electrlysis.
