Im lost too, are you saying you can only fit 2.5 cubes?
If thats the case you can either go with (1) SSD ported or a pair sealed. With the power you have, plus limited space (im asuming its going in a mustang), I would go with the pair sealed. SSD sound really good sealed or ported.
i did this exact setup, only with 10s, and it wwaannggeedd. i suggest the copper coils over the flatwinds. much better sound. ive used both the flatwinds and the coppers. good stuff.