Dart/Barracuda gas tanks are the same so I would assume the spare tire well will be as well but only one way to find out
The tank when installed would be flush making your trunk area flat.If you Google image 67 Mustang gas tank installed you will see what I mean.It would make for a clean install IMHO.
Actually the tank will fit the other way but you would have to run the filler neck tube down the inside of the quarter panel back to it which may not be a bad deal either because it would free up a some more room.
If I remember right the sending units had the same Ohm range so the fuel gauge will read right as well.
If you need the tank just let me know. It's old and rusty but will work great for mock up.