Just depends on how big of a music listener that you are. I love music so my stereo in my 69 is going to be good...but not over the top.
Put a head unit in the stock location. I got last years model of the Pioneer Premier series DEH-p650. It has 50watts a channel and has good controls for the subs. I am going to run 2 4 inch speaker in the dash where the factory speaker went and use the radio power to run those. I am putting 2 6.5 in the kick panels where Steilow did his in the Mule and I am doing them the same way. If you are not running vintage air, you can get a 5.25 to fit where the stock kick panel speakers fit. Running 2 6 by 9's in the rear and 2 10 inch subs in the trunk. Using the small Memphis Belle amp. 50 watts by 4 channel for the 6.5's and 6 by 9's. It has a sub channel that I am gong to run on a 1 ohm load on the 2 10's and that will be 400 watts.
If you do not want to go that elaborate run 4 or 5.25 in the kick panels and 6 by 9's in the rear with a 4 channel amp. You can get a "Bazooka Tube" with its own power for the trunk...you really need a sub.
www.crutchfield.com you can pick up some good deals.
Hope this helps.