id say with a 4channel radio, the 2 rear speakers would ideally be behind the occupants and seperated noticeably right and left, so centered wouldn't be the best idea.... however with 4" speakers, i can tell premium stereo quality is not the goal, just something obviously better than stock.
would you be able to post pics of an el camino interior cause off hand i have no idea of the stock interior setup/location of seats etc.
i know this is the wrong year el camino, and very custom, but it gives an idea as to where id suggest locating the rear speakers... the ones in the upper corners. i wouldnt worry about sounds being muffled, also i would recommend 6.5 speakers.
http://image.chevyhiperformance.com/...o+Interior.jpg