Some good ideas in there. Are those two 6 inch subs in the tool cubby? I don't think I could get a second 8 in my design, but a six and an 8 would give a pretty broad frequency response. That center console setup with two sixes is sick, but seems like overkill for 6 inch subs. Seems like you could get a single 10 in there and get better response on lower frequencies.
Two 8s in a box or simply mounted to a cover plate? I tried one 8 with a cover plate and was not very happy with the sound quality. I just don't think an 8 makes a good free air speaker. I guess you could make enough room if you used molded fiberglass.