Seeking advice on setting up system in van

[batook]

I've been driving my parent's old van a lot recently, enough that I want to put a system in it. I've pretty much inherited it from them anyway since I do all the mechanical repairs and maintenance on it now.

Its current system is stock, and the speaker locations are weird. There are 5.25's and tweeters in the dash, and then another set of 5.25's in the rear panels. I can't hear the rear speakers when the fronts are playing: fader at "flat" and fader at full front sound the same.

I have some old gear in the garage I can use, but I could also buy some new stuff to use in conjuction, or upgrade/expand later. I'm planning on using an old Lanzar Optidrive 160W x 4ch amp. I have a set of Infinity 5012i 5.25's that I can replace the front 5.25's with. I'm concerned that I wont be satisfied with the bass response unless I put in some 6.5's somewhere though, like in the front doors. I have some Kicker Resolution 8" midbass woofers, which are 2-1/4" thin and can be installed in the front door panels, or in the rear panels to replace the 5.25's back there. Just not sure if I'll be able to hear them much from the front. I don't care about the rear passenger's listening experience (I never have any).

I was also thinking I could buy a 3-way component set, and install 6.5's in the doors, 3-4" midrange in the dash where the 5.25's are now, and tweeters in the side pillar. Then maybe just run the 8" woofers in the rear from 500Hz and below for rear fill. But again, its so far back there I'm not sure I could hear it from the front, in which case maybe I don't need anything back there. Or throw in some 6x9s?

Most of my installation experience has been in little 2-door coupes, so having all this length and airspace to work with is screwing with my head and making it hard for me to decide what should be done.

No plans for a sub yet, but that could change. I usually have no rear seats in it and use it like a truck to haul stuff, so losing any of the cargo space to a sub is a concern.

Anyway, any advice on what would sound good is appreciated.