1st of all check you have blind rivnuts, see here.
As mentioned blue hylomar will seal the pump/gasket/tank interface. Do not overtighten the fixings & do up gradually doing opposite fixings to spread the load.
A drop of araldite under the rivnuts is a good idea to stop them rotating. I reckon when you get the bolt out the rivnut can be fixed, in situ, with some araldite.
To get rid of petrol vapour fill with water, several times, drain & leave to dry in the sunshine......................