Just curious and I know - None of my business - but I have to ask - Why the heck would you want to do that?? If you do and are are taking a poll - I would vote for a trunk location. Just an opinion from a lifelong 68:GTX owner.
As you compared it to the Charger, I would suggest seeing if the Charger tank and fittings would work for you since they're both b-bodies.
It's just personal preference I guess. I love the look and convenience of the quick fuel flip caps.
Well that’s true - nothing convenient about the behind the license plate gig - but you have to admit it’s pretty funny when you let someone else gas her up and they are at a loss trying to find where to put it.
Behind the license plate wasn’t that bad when we had those Hi Jacker air shocks on our B body’s. Lol 
There is a lot more than just bolting parts. Better measure a charger to your GTX. The charger has a different angle and is very wide. The quarter is not just a hole. It is raised and molded to fit the ring. So there is a lot of fabrication just on the quarter. Plus the parts. Tank, sender, filler tube, flip cap and bezel, vent tube, plus misc. Then the tank may not fit the trunk bottom without doing some fabrication of the mount bosses that position the tank. Then you may still need straps, bolts, top tank insulator pad. To me it will be a big project with a hefty cost. To me it is a waste of time and money.
I have a '64 Polara that also has fill behind the licence plate. It does not matter what side of the gas pump you pull up to. A real plus at busy gas stations.
Yup!
Good point.
Thanks Kramer. Great info!
Well, yes and no. You're never stuck while you have the imagination and skills to bring your ideas to reality. I went the fuel cell route and placed two motorcycle style pop up filler cap/necks on the quarter tops next to the front corners of the trunk lid on my 67 Coronet. They are from Rullo Caps and have an aluminum body with a weld to neck to attach to the quarter skin. I routed the filler tubes down and through the body of my trunk lid gas lift braces. Easy peasy. Look tough too. Now, to answer your question, yes, it's likely that you can modify the stock tank to have the filler neck routed up through the floor in the area behind the wheel well hump inside the trunk and curve forwards and up to meet a filler neck on the trunk deck. I would investigate the use of all, or parts, of a 68/69/70 Charger tank and filler neck assembly. Good luck!
