Daniel,
A poly tank needs room to grow (from exposure to gas) initially. The tank size was selected to allow that during the design. I would not think that they would have just thrown the tank in the well and closed the lid although we've seen some weird things. I would think that it has or had a bar across the tank to keep it from moving much. When I converted mine to having a belly tank I left room around it for 2% expansion and installed a piece of composite across the top in the spot designed for it. I don't think moving around a little when empty is a big thing. It needs not to be slamming around when full, for sure. The tank has done it's growing by now so you could wedge some starboard around it - make sure you don't create a puncture possibility. I did the same thing on the 40 gal water tank I added to my 230, which I documented here. I didn't care if it moved a little, I didn't want it to move a lot.
