I went with a Pioneer setup. Fit perfect behind the seat of my 78 and i can still push the seat all the way back. I bought mine off buy.com though as it was a lot cheaper
SUB:
http://www.crutchfield.com/p_130TSS2...S2501&skipvs=T
BOX:
http://www.crutchfield.com/p_130DSW1...UD-SW100T.html