41599
I believe you will have to use 41599 - Unlisted procedure, tongue, floor of mouth
40804 is specific to the vestibule of the mouth - The vestibule consists of the mucosal and submucosal tissue of the lips and cheeks within the oral cavity, not including the dentoalveolar structures. (from lay description of 40804, Encoder Pro).
However, you can base you fee on what you would charge for 40804.
Don't be afraid of unlisted codes. I use them all the time. Yes, the payer will require op notes and some explanation of your reasoning in re the fee charged, but I don't usually have any problem getting these paid.
Hope that helps.
F Tessa Bartels, CPC, CEMC