Hi Oscar,
You're really getting into the swing of things now.....
Capping the end of chassis tubes is fine (stops the ingress of water). I TIG welded end caps to all my open chassis tubes which were open to the elements.
By what you are saying you have welded nylocs to the seat/harness plates.
To be honest that's not a good idea. IMO..
The reason being,
1, You don't need a nyloc nut as harness/seat belt bolts have a shoulder on the bolt to stop the bolt being tightened onto the harness/seat belt bracket. This way it allows the bracket to swivel on the bolt.
2, It doesn't look good to Mr IVA man (big gap where the nylon used to be if you get my drift)...
A standard nut welded to a plate will be fine for this. Just take a photo of the bolt with applied "Thread lock" for the "Men In White Coats" just to keep them happy....
The same applies throughout the build, for instance if you have to grind a weld off on a critical area ie.. harness point plates to transmission tunnel box section (for flush fitting of ally panels) make sure you have prepped the edge of the plate for the weld to be ground off this will show them you know what you are doing so to speak.
Johno