Possible for track car? Welded plate over the top with sender in?
You could probably get an ally tank fitted into the wheel well but I can't see any advantages of doing it.
Adding weight behind the axle line isn't good for handling and a fuel tank sited just behind the bumper can't be an improvement in safety!
The standard tank position is pretty much ideal, ahead of the rear axle line, nice and low down in the bodyshell and the tank itself has a fairly shallow profile.
Unless your car requires an ally tank for some form of motorsport, I'd stick with the standard one.
HTH.
I agree. I can't see the point unless you're building a rally car and need to move the tank so it doesn't hit things.