fit the rubber gasket on the inner rim of the tank first and slowly slide the pump in place. the pump has a spring on the bottom that should stay if you installed it right. it should be a very snug fit.
i made the mistake of trying to fit the gasket on the pump and attempted to force the pump in.
it seemed like it worked but it didn't
i found out the hard way the first time i filled up yesterday.
the gasket keeps the fuel from spilling over when full. now the right tank is always full as our suction system pumps fuel from the left to the right continuously.
my car port has melted asphalt from the gas split overnight