I would worry about steel rivets rusting as they aren't painted etc. Aluminium rivets are plenty strong enough, they are used in planes! Make sure you use closed end rivets
I don't like rivnuts, they have a horrible tendency of spinning with the bolt still in them
I ended up tacking the rivnuts to my chassis where they hold the nosecone on. The only other body part held on with them is the scuttle.