I have just spent an afternoon with a slide hammer trying to get this thing to come off the filler pipe and all I have managed to do is get it to move about 1/4". I can tell it is a rubber type fitting as it moves with pressure but won't actually come up. The overflow tube that goes under it is angled which seems like about the worst decision ever (maybe it turns?). Does that mean the whole thing has to turn while coming out to get the pipe out? Am I missing something here? It's the big NSS labeled part in this diagram.

 

So thinking about it I went back and started wiggling it back and forth prying a bit with a curved seal puller and about an hour later it wiggled free. If you need to do it in the future I would say skip the slide hammer or any sudden force and put instead pressure back and forth trying not to damage the sheet metal.

 

As soon as you get it up a little, start squirting some lubricant WD40 or similar underneath. Right at the outside seem.
Absolutely flood it, then let is sit and soak in. I ruined one cap trying the twisting. Method
Luckily I have 4 extra caps. ( now 3.)

 

Even applies to Coupe's - I had to use a piece of wood as a load spreader, introduce a two legged 'internal' puller under the lip and lever the puller up with a pinch bar and even then it was hard because the neck wanted to go back in as soon as I released pressure, took a lot of pressure to remove and would never have moved by hand with the twisting in any direction other than round and round - that O ring in the tank gets a good hold especially when there's any kind of build up on the neck, my experience is that the lubrication simply doesn't get under it but once it is moving it does - make sure the rubber sleeve is also loosened off on Coupes - not sure about convertibles.

 

I read somewhere that a strap wrench plus WD40 works wonders on removing this item.