I worked at Westside Car Audio in Perth WA for a few years and my advise is to face your subs towards the back of the boot " takes up alot of room but hey" the bigger the box the lower the bass, the smaller the box the more punchy the bass, so if you like Drum and Bass you would get a 12" or bigger and put it in a big box, if you like Techno/anything you would put it in a slightly smaller box.
I fitted 2 x 12"s into an R33 one time, faced them towards the back, bridged down 2 about 2ohms, Dont know if you can even get them anymore.. 12" Earthquake MAGA series subs, chew around 500RMS each but can take a load more!
Well anyway we found that the skylines respond better if the subs face backwards. Over 138db without a proper tune isnt that bad in a R33
PM me if anyone wants some help.
PS: Ported boxes are designed to give you a lower bass however they reduce your hard hitting punch " incase your reading this Derek "