With my 79, I had the stereo in the glove box, mounted with some pieces of wood screwed thru the sides of the plastic. I just used the cage that came with the deck to hold it in place. As far a speakers, I found some surface mount 3-way marine speakers and put em in the kick panels, and of course 2-10s behind the seats.
Wiring I ran all custom/new hard wiring.
