Adding more speakers won't make the current speakers sound better. I don't think putting speakers in the middle seat or under the dash would sound good. No need to go crazy with big speakers and amps. One set of quality components and a small amp is plenty. Then add a subwoofer later if you need more low end.
Kick panels with properly aimed speakers is the best location. A-pillars would be the second best option. Check out par4tom's kick panel build.
http://67-72chevytrucks.com/vboard/s...d.php?t=730495