I have 40"x 15 x 25 to work with. Would this be sufficient for either 4 sa-8 v2's or a single dc lvl 6 18" assuming it'll fit physically. Its going in a 2005 civic sedan with a 220-250amp alt, 2-4 xs d31's and probably a stetsom 7k or similar. I would like to have itforward fire into the cabin while being sealed from the trunk.

Well that is roughly 7 cubes gross. For the sa8s you will need around 76 in3 of port area. That should be plenty of room considering the sa8 only needs around .75 ft3 per sub.

Download torres box calculator and play with the numbers.