are you going to fiberglass the wheel well then attach the box to it? More info please... packing peanuts are commonly used to determine enc. volume... be sure to read some of the stickeys and search this forum-- lots of answers available out there.
I am gonna do something kind of like this.. using the wheel well, then, instead of flush mounting the subs they will be slightly raised above the level of the floor for "aiming" purposes..... something kind of different that I have not seen done (the way I have pictured), hoping it works. Good Luck