You're right knuckle dikk, I built the whole back seat. Nice try. Sorry, @Caliboy916 this guy likes to ba a D-bag to me.You do not. Though it shouldn't matter. The only reason you would want to worry about that is if some subs were mounted inverted and others not. It is often worth trying reversing polarity of subs regardless though just to see which phase blends better at crossover point to the mids. Normally there will be little audible difference.
That said, at least one of the chucklefucks here has never even built a box in their life. 7 cube isn't too out of the ordinary per 15" sub. You will gain more output and efficiency from going with a big box (at a point you hit diminishing returns) but you will need to be mindful of hitting mechanical limits of your subs. If you know your subs perform well in a large box I don't see a problem.
Similarly with ports, you will always gain from a larger port though there also you hit diminishing returns and eventually hit physical impossibilities. Round port is most efficient, and flared edges also add efficiency, but rectangular or some other shape may seem more practical when you start putting things together. Provided you follow the fundamentals of port design it shouldn't be terribly dramatic which way you go.