With that airspace you will be okay as long as your throwing them quite a bit of power. I would recommend tuning a bit higher for those subs / that box though. More around 40hz.
i already get a deal on them seeing how i worked at a shop who sells them . i have one already . carbonfiber cone and cap and extra stiff option. i also own on ab400.1 i plan on getting another woofer and im trying to get a 400.1 off here but the guy hasnt wrote me back. so we're looking at about 4000 watts a woofer grated i can fit the 4 kinetiks 2400s in the back without the car breaking .
Ive gone from a 3 cube box to a 4.8 and ive noticed that the 3 cube had more spl but the 4.8 is lovely on music and lows //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
You should be fine I had the same driver 9518g with carbon cone and cap in 6 cuft with 2 - 6" aeroports tuned to 34 and was VERY happy with the output. Wish I still had it. 6ft seemed to be the sweet spot between daily low output and power handling, at least in my application. build it and no one who hears it will tell you DD sounds bad anymore!