It would really depend on the current condition of the interior (for speaker sizes). The doors could already be hacked (hole drilled in them) for a speaker mount location; in this case you would install what ever fit in the door. For your information, there was only 1 speaker in these trucks. It was located in the middle of the top of the dash, facing upward. Only an AM radio...
The kickpanels are a good spot for a pair of speakers. Pods are already made aftermarket for them (scroll to bottom of link)
http://www.lmctruck.com/icatalog/cb/full.aspx?Page=110
Also on this page you will see so options for the center dash speaker location (dual voice coil units).
The fuel tank is inside the cab behind the seat. This doesn't leave you much room. Rear 'fill' speakers are almost impossible, there is just nowhere to put them. I was successful at placing a 10" sub underneath the factory bench seat, and mounted my amp in the little available space on the passenger side of the cab behind the seat (the fuel tank leaves you about 16" of free space on the passenger side only). If you get a Blazer tank and mount it underneath the bed of the truck, you have all sorts of room to mount crap.
Your speaker wire and amps depend on the size speakers you use. Also the factory dash is for a knob style radio, but can easily be cut out for a din sized cd player. Because of the design of the dash a 1.5 din will not fit unless you did some metal fabrication.
