1990 Corolla DX Gas tank leak

I have a '90 Corolla and lately I noticed that when I tried to start it I would have to crank the engine for longer than usual maybe like 5-10 secs, it reaked like gas and got horrible gas milage. I did some looking and testing and found out that the gas tank is leaking from the top. I am not sure yet if it is rusted through or if the gasket around the fuel pump is leaking. I am hoping that someone here has had an experience with this and can give me some helpful advice. I am going to have to drop the tank, but don't know what I am getting into. Anyways, any help is appreciated.