I know that there have been a lot of suggestions here, but I would like to share what I did with my 84 C20.
I kept the original sized head unit cutout and installed a retrosound model one. I had trouble finding some 4x10" speakers for the rear, but crutchfield had some Kicker brand ones that work great. As far as the front, I used some infinity 3.5" speakers and used 4. One each in the original cutouts and then ran two more in the center opening. Once it was all said and done, I still didn't like the amount of bass I was getting. I'm not one to want to rattle everything.....just wanted some good sounding music. I installed a Kenwood powered subwoofer (150 watt) in the center of the rear of the cab. It was exactly what I needed. Overall, I couldn't be happier. I utilized all the original openings for speakers and didn't have to hack my dash at all.