Nope, the space the person is willing to give up is the first determining factor. If he can only fit 8's where he wants to put his subs then that would narrow the options down a lot. If he was willing to give up his back seats and do a wall he would have many more options, such as two 15's or four 12's, etc. What i'm saying is you can't just decide you want an 18 but then you find out you only have room for a 12 in your available space.
Yes i know, the OP said he wants to get 8's under the seat since that is probably the only option in that case.
OP, i recommend as many sundown sa-8's you can fit in there. They have proved themselves to be great subs at a nice price.
That was my second guess.. I was looking at this http://sundownaudio.com/index.php/rides/customer-rides/item/cody-eastman-2000-sierra.html?category_id=9
Shit looks nutty..