Yep
Hi Rob M,
I have indeed replaced the fuel tank on my own E39 during a complete engine room refit about 3 years ago.
If yours is at all like mine, the only way to remove it, short of cutting it apart, is to remove the engine and all the other stuff around the tank first. Then it can be lifted up, turned 90 degrees, and taken out forwards right through where the engine was, into the cabin.
I handed my leaky, rusty original tank off to a certified welder, who used it as a model to create a new tank of the same dimensions out of Aluminum. The new tank also has more baffles inside and an access port on top, which the original did not have. So far it is holding up very well.
Here are some pics:
The first is what the engine space looked like right after removing the engine (we dubbed it "The Bog of Eternal Stench"). The red mass in the back is the old tank, which was mostly being held together by the rust.
The next shot is what it looked like after most of the mess had been cleaned up. You can see the molded cradle where the tank sits in the back there.
The third shot is the glorious new tank in place on its righteous throne.
The last photo is how everything looks when put back together.
I hope this is useful info for you. For me this was just one part of a much needed complete refit of the engineering space. The engine was coming out anyway. If your engine room is OK and the only problem is the tank, you will need to either remove the engine as I did, or you may be able to take a que from evm. Perhaps he has some pics of what was modified in the bulkhead to allow the tank to come out.
Good luck, and feel free to contact me with any questions!