typically how much cubic feet do bracing take up and the port.
That depends on how much bracing you use and how big the port is.
do subtract the about of wood used in bracing and the whole port (If concidered a solid) from the over all cubic feet
Technically yes. You can figure out every cubic inch and subtract it to get the net volume. Or you can build a box 1 cubic foot bigger then needed to allow for the displacement of the subwoofer, bracing and ports. In your case you know the biggest internal volume you can build is 3.5 cubic feet. So find a sub that will work in a slightly smaller net volume.
also can you expand on the choices of drivers?
Off hand, I'm not sure which driver would meet your needs. Hopefully someone else who is familiar with 12 inch subs can make a recomendation. You can download WinISD and do some simulations if you're inclined to.
http://www.hometheatershack.com/foru...30-winisd.html thank you
Anytime.