When I had my 72 for sale I started out trying to get $8-$8500 out of it, which was about what I had into it. I had gotten what I felt was a great deal on the initial purchase of the truck, and with the stuff I fixed/upgraded I had about $8K into it. It was a nice mostly original truck with low miles. I still have pictures of it on the net at
http://www.tmcarphoto.com/72 where you can see the condition. I had it for sale for a couple years and even went as low as accepting offers of $6500 for it, from buyers that never returned with the cash.
Finally I traded it to a buddy for a car. He put a different rear bumper on it and a tonneau cover, and a woodgrain tailgate panel on it (this was shortly after I had painted the center section of the truck white). He then took it to Goodguys in Indy and within a couple hours had sold it for $10K! It was probably in similar shape to your truck, maybe not quite as nice.
Regarding your truck, I would say this: Subtract all the money you have in the stereo system and see where you're at on price after that. As nice as the stereo stuff is, 90% of the people interested in your truck won't care what it is or how much it cost. Sad but true fact. Once people get tired of seeing it on Craigslist you won't get any action on it, so try moving it around to areas that are still near you, but don't have the same ads. Sometimes people will drive considerable distances to find a ride that they can't find locally.
Incidently, I traded my buddy's car for a brand new $11K motorcycle, which I sold for $8500 cash. He had his car for a sale for a long time at $8K and couldn't get a bite on it either.