if you can only fit a 6.5 you could go with a digital designs W6.5 or and adire audio Koda 6. but i doubt you get enough bass out of a 6.5". if you could squeeze in an 8" or 2 i would definaley recomend Resonant Engineering RE 8's which are only $49 a piece and are pretty potent.

http://www.adireaudio.com/mobile_audio/drivers/koda_6.htm
http://www.ddaudio.com/caraudio/woofers.asp?series=W6.5
http://respl.com/8.html (prise shown is msrp, real price is $49)