If the tank comes out really clean there is no reason to use a poured in tank liner. I would not reccomend using a drain plug with a coupler for the fuel pick up. In reality there should be a gallon or two of fuel that is not usable below the fuel pick up. The sediment in the tank will sit at the bottom. If you make all fuel usable then all the trash that is pumped in at the gas stations will plug up filters and possibly go through the fuel system. also condensation will stay at the bottom and not be picked up if a few gallons are not usable.