Mine is still a work in progress, but here's my plan:
I found a manufacturer of repo head units that look like the factory unit but with modern internals for about $550.
http://www.classiccarstereos.com/196...uck-radio.html
For speakers I grafted in some speaker grilles in the dash and rear cab corners. The pattern is modeled after the pattern of the dash center speaker. They will accept 4" x6" speakers.
Finally, I plan to install a simple compact powered sub under the seat to help with the lows that will be lacking with the 4 x 6's.
http://www.classiccarstereos.com/sel...ubwoofers.html
I had the luxury of a water jet to cut out the grills. They were time consuming to weld in but in the end they look factory. I also used a sheet metal slip roller to match the contours of the dash and cab corners. I did all this to get the same result as you are looking for; a modern system that looks factory and hidden at the same time. Time will tell how well it works. Not a killer system but should sound decent.
Soon as I get home I will upload some pics.