What I have done regarding the center dash speaker location...
http://67-72chevytrucks.com/vboard/s...07#post2557107
I have used a variety of speakers stuck under the seat (mostly 6x9 refugees from another project).
The latest incarnation for rear speakers are a pair of 5 1/4 speakers from another project that I have atop the gas tank stuck in the corners.
Then I placed under the bench seat a Kenwood shallow mount sub that I got from Crutchfield
http://www.crutchfield.com/S-GtpUSX3...KSCSW10&tp=114