I have recently been working on a Classic Rally car. It has twin tanks with flip open gas caps. Depending on the amount of paint on the tank neck when driven on these brass rings can be tough to remove. I keep a lot of scrap material such as the waste from making my own wheel spacers. The scrap from hole sawing the centers was just about correct once touched up on the belt sander. The 5/16 hole in the center aligned the puller. No rocket science just having stuff and knowing where to look for it. Steve (CTR) 

You guys do like to make mountains out of molehills .....

Why not try the easy stuff first.  The cap might require a gentle tap (rubber hammer) if its done up very tight..

All you need to remove the collar is a two arm puller and a small bridge for it to work against.  You could get out the lathe or CNC machine to carve a custom billet stopper .... or you could get a small piece of scrap steel  (mine's about 1/8th thick and maybe 3/4 inch wide) cut off the appropriate length with a hacksaw and smooth of the ends with a file....  Whack a small divot in the centre to keep the puller in place.

Of course the PO MAY have welded it in place and you MAY need a laser cutter to free it.... but have a good look before going nuclear.

Cheers, Ian

I like this idea Steve, I have access to a lathe for making a plug, perhaps a bit of heat gun as well. I will eventually post my results. Thank you all for the ideas.

I place a 5/16 thick circle cut from aluminum stock that just fits inside the brass ring the I have a smallish three jaw puller and with a little help to line it up it pulls the right off. Steve (CTR)

The heat gun is a good idea and if you don't have one Harbor Freight sells them cheap.

Those brass collars are put on by expanding them with heat first. There shouldn't be any glue.
Even if there is some glue or something, you should be able to heat it up enough with a heat gun to soften it up. You probably wouldn't be able to get it heated up evenly enough with a soldering iron. If you don't have a heat gun, this would be a perfect opportunity to go buy a new tool. They are super handy for lots of things.

I have looked for the allen grub screws but have found none, I assume they would be on the cap collar, not the brass threaded one, but if so they could be under the rubber body grommet. Heat is tempting and I have considered using a large old time solder iron, like from the roofing trade, to apply the heat without a flame present but there is not good flat area to transfer the heat. At least it's not a rush to decide as everything is working, but I did restore an original gas cap to like new and would like to use it.

I'll bet you could just use a heat gun or a really hot hair dryer to warm it up and then twist it off. You might need a big set of channel locks pliers to get a good grip on it.
I really doubt you could put enough force on it to twist the neck off of the tank, unless you are built like the incredible hulk.
I would NOT use the propane torch idea unless you have a camera running and you want to get famous on YouTube.

I don't know if yours may be the same but the ones i have seen and encountered have 3 allen head grub screws locating it around the edge

You won't like my suggestion.  I don't even like my suggestion.

Drain the tank and disconnect the fuel line from it.  Cap the tank nipple where you disconnected the fuel line.  Fill the tank with water all the way up to the bottom of the filler neck.  Protect the car body with multiple layers of body repair fiberglass cloth.  Then use a propane torch on low heat to slowly raise the temperature of the neck.  Periodically try to twist the Monza cap to unscrew it. 

If the PO used something like JB Weld to anchor the parts you will have to get the temperature up to about 600 oF and hold it there for a while to start breaking down the epoxy.  If you are lucky the PO used something like a 5-minute epoxy which will break down at a much lower temperature.  

I am going to need to remove my gas tank tank at some point. In order to do that I need to get this gas cap off. So far I cannot twist the cap from it's threaded brass collar. Instructions for the cap installation say's the brass collar is pressed and glued on to the tank. it's possible the PO used epoxy for installation and may have used it to also fix the cap in the desired position. I hate to destroy the cap, but I could split the AL cap collar from the brass collar. Not sure how much force I can use to twist the cap without ruining the tank. Any better ideas?