Be very careful. The moment you take the deck off, the ship will settle into a different shape, and when you re-fit, you will be re-fitting to that shape. The deck may not go on cleanly again, and the hull will be stiffened by the re-fit to the new shape.
It may not be a problem with a relatively small ship like a 21 foot, but my olde 36 ft ??? I would not risk it, even though I really would like to get the old tanks out of there.