I would say if extending a couple wires and fuel lines sounds high on the difficulty meter, I might rethink the project... (That may just be the way I'm reading your post.)

Beyond that I would personally look into a wagon tank or just get an aftermarket racing-style aluminum tank made to be installed on a truck. I don't know what the sedan tanks look like, I'm realizing, so they may or may not be able to be mounted flat. Either way it should definitely go under the bed. I suppose another direction to go would be behind the seats like some pickups.

I'm all for it though, I think they look good when done correctly.

-Rog