You're going to need to gain access behind the seat, either by moving it forward or pulling it. The filler neck is about 4 inches long, and attaches to the tank via a heavy wall rubber hose, and 2 clamps - one near the tank and the other near the top. The upper one can be difficult to reach, but if it's the original you it should have a 1/4" hex so you can use a wrench or small ratchet.
On your truck it's likely one of the clamps is loose, or missing - or the hose is so old it's bulging so much it's not holding the filler neck in place. If this is the case you will need a new hose. Truth is, if it's original it needs to be replaced no matter what since 40+ year old gas hoses deteriorate.
It may take some effort, but once the clamps are loose the filler neck just pulls out. It's much easier to replace the grommet with the neck out, although some have installed the grommet with the neck in place, using string like when installing a windshield.
