I would leave it in, it might also be a structural member as well as helping the gas from sloshing all around.
Find a local chemical paint/rust removal company the can soak the tank and de-rust the inside of the tank. Then coat the inside. It costs me about $120 to do a 20 gallon tank and they come out looking new, the chemicals don't hurt the original tin coating or lead seams at all.
My .02
Mike B
