The tank definately needs to be off the floor--not just chafing, but to keep it from sitting in salt water (the reason that aluminum tanks fail).
From David Pascoe:
http://www.yachtsurvey.com/fueltank.htm
"you are going to set the tank on top of 1/4" strips of plastic about 2" wide and spaced about every 12". Its best to place the strips transversly to the length of the tank. We recommend Haysite or any of the fiber reinforced plastic sheet. The strips should be cut to a length exactly 1/4" shorter than the width of the tank. We don't want the strips sticking out the sides and collecting water.
Apply the adhesive so that the entire surface of the plastic is coated with 5200 and will not leave any gaps or crevices for water to get into. Once the strips are pressed into position, make sure that the ends are equally 1/8' from the edges of the tank. Wipe off the excess 5200 that squeezes out, making sure that the joint between tank and plastic strip is completely sealed. Don't leave any globs of material.
Allow 24 hours for the 5200 to set up before setting the tank in place. Next, apply two 1/4" wide beads (like a stream of tooth paste) of 5200 along the length of each plastic strips that are now glued to the bottom of the tank. The strips do not get fully bedded because tanks expand and contract as they are filled and emptied. If the bottom distorts, we want the plastic strips to loosen from the deck, not the bottom of the tank."
Mr Pascoe also states:
"Do not use rubber strips: rubber contains carbon, which is cathodic to aluminum and will cause galvanic corrosion. "
All of David Pascoe's threads are definately worth reading, even if you think he is too "strick".
I don't know how the tank is put in place, but with the top off, it might be well worth while to take a look at it, how it is fixed in place and to make sure it is properly secured. I would think that the inspection plate should be over the sender unit, and the draw tube also be easily accessable.
Bob Austin