If you don't drop the tank, you need to remove the filler neck completely. Or you can drop the tank and leave the filler neck hanging, or remove both (probably easiest in the long run, and lets you inspect everything for condition problems). Consider: maybe it's not the seal, it could be rust holes in the top of the tank — that would be much more common. The seal is pretty robust — I wouldn't expect it to go bad if the trunk floor seal looks good.
