What about putting a synthetic rubber bladder inside your exisiting tank? Some time ago I read about a company that will make a bladder to your exact specifications for a few hundred dollars. I will see if I can't dig up the name of that outfit.
I'm not sure how they deal with any baffles that may exist in the metal tank.
Also, you'd need a large enough opening in the top of the tank to work the bladder through (I think I read that 8 inches was enough).