Starting in model year 2003, the tank was upgraded to an 18 gallon tank. I would think the housing specs would be the same. The black tank I believe sits for the most part under the raised platform the head sits on.
So, I'm thinking that yes, you might be able to fit the factory 18 gallon tank in the space the 8 gallon resides, but I would contact the factory just to make sure. They can provide part numbers, etc as well.
Computers manufactured by companies such as IBM, Compaq and millions of others are by far the most popular with about 70 million machines in use worldwide. Macintosh fans note that cockroaches are far more numerous than humans and that numbers alone do not denote a higher life form. -NY Times 11/91