90 dodge truck, box, system advice

I just picked up mint 1990 Dodge dually & there is a decent amount of space behind the seat. I am thinking of going with either 2 10's, or 2-3 8's & figure if I mount them firing down I can get enough airspace while keeping the seat all the way back. I will then mount 2 amps under the seat, one for the subs & one for the components.
What sub specs should I take into consideration for this? Any recommended subs?
Would it be best to mount the subs parallel with the floor, or have them tilted slightly forward so they are angled towards the front slightly?
S.

In this setup I usually stick with two 10" subs.  Sealed box.  The type of subs come down to how much bass you want, and what subs you can physically fit underneath the rear seats.  My customers always want the 10W7's under there, but with 8" of mounting depth, it just ain't gonna happen.  You don't have to angle the subs forward at all.   Keep the install simple and fire them downward. 

I recently picked up a mint vintage dodge truck & am trying to figure out the best system to put in it. This truck is a keeper so I plan to do my cleanest install ever! This one is for sound quality.
Its a regular cab so there isnt a ton of room behind the seat & I dont plan to lose any. I was looking at downward firing, but now I am thinking of 2 8's, one mounted facing the left side of the cab & one to the right. Kind of a homemade double bass tube. That way I could still throw a jacket or something behind the seat on top to the box. Would they interact negatively with each other in that configuration? Otherwise I was thinking of using a single 8w7 facing the left side of the cab, & running a port to the right side.
The truck comes with 6x9's in the door & 4x10's on the side behind the seat. Should I just replace these with better units or should I get some components in the door? There is lots of room in the door, but I want to leave it looking mostly stock.
The amps will mount under the seat, or perhaps on top of the box.
Any other suggestions for what type of box?
S.

For the 8 inch subs have you ever considered using tang band subs?  Go to partsexpress.com and check them out.. Great bang for your buck...  Just an idea...