Some small tasks needed to be done before closing the tanks.
First I tested the friction of the rivnuts of the new left tank. I want to torque the M4 bolts for the fuel sender with at least 20 inch/lbs. So I checked if they remain in place when applying 25 inch/lbs. Unfortunately almost all start spinning long before this value was reached. I have to redo these, which is easy now since the fuel tank is still open.
Next I looked at the ghost hole which I have in the right tank but not in the new left tank. I think this is a revision update. Since I want both wing to be symmetrical I transferred the position of the ghost hole to the left tank: