A little known fact is Sammy Miller's "Oxygen" rocket dragster used a two piece frame which bolted together behind the cockpit. This was an adaptation to the shop where it was built being too short for the full length frame.
Dunno. They used bolts and rivets to hold the SR-71 together, and it was fairly reliable and pretty fast.
Welds are only safer than fasteners if you don't use the right fasteners.