There are a couple of more things that needed to be done before closing the tank.
Since the rivnuts in the new left tank spin before reaching 20-25 inch/pound I removed them. I grinded from a piece of aluminium a special scraper and removed the tank sealant around them and cleaned the area with SkotchBrite and Aceton:
Another thing I noted is that the float is not moving freely and can be positioned at any position, this could cause a stuck fuel sender:
Peter V. advised to open the sender, like he did and modify the spring a tiny bit.
It only takes 2 screws to open it: