planning new setup

Yup, use one single 10" or 12" driver. Your amp should be stable to 2 ohm mono on the sub channel. (check to be sure though). If so, look to the Pioneer Premiere shallow mount driver due to the rather limited depth available for a proper enclosure behind the seat. Ignore the need for kick panels. If you want to keep it simple, put the tweeter into the stock 3.5" location on the top corners of the dash. The midbass driver can easily go into the door, about 5" back from the front of the door, down low where the door indents. It must be 5" back from the front as that is where the steel reinforcing is behind the sheet metal. It is a rather simple process to make a small panel to attach to the bottom of the door and properly mount the speaker there.

As far as the rear speakers go, there are aftermarket 6x9" mounting adaptors available for this vehicle. American International I believe make them, they should bolt right into place.

For the amp.....take back the Soundstream. The Alpine is actually rated at 55w rms x 4 but produced way more power than that. It is also way better built than the Soundstream could ever hope to be. If it were me and I came a cross a amp like that 3555, I would be looking for a matching old school amp to go with it.