That used to be a standard practice until many of the aluminum tanks developed poultice corrosion. Aluminum needs air to protect itself from corrosion. Best way to mount is to glue 2"x1/4" fiberglass battens spaced 12" along the bottom of the tank with 5200 - making sure to completely cover the batten and seal the sides of them. When these are set, liberally apply 5200 to the bottom of the strips and set the tank down in place on a flat mounting surface - effectively glueing the tank to the boat. The tank won't move after that, but you can also put some plastic chocks on the corners to be sure. Air can now move all around the tank, and the 1/4" fiberglass strips keep the tank away from any nominal moisture. Mounting on neoprene is tricky - you MUST have pure neoprene. If there is any carbon containing rubber components in it, they will destroy the tank.

Are you sure your old tank is aluminum? 0.09" is very thin for aluminum and is the normal thickness used for monel. Monel was inexpensive and common as tank material 26 years ago. If it is monel, you have a lifetime tank that is superior to anything you will get made today, and I would not change it.

Mark