[BIGglaSS]

My opinion of the undercover speakers, is they will sound like a glorified factory radio, pointed at your ankles. But, they have there place.. Where you just want to listen to something, and have no where to properly hide speakers. I think you could do better with rear corner speakers for less than the $ of undercover.

For now, don't worry about # of channels or watts, etc. Lets nail down the type of system you want. You can always add as you go.

Channels are the number of amplified outputs going to speakers. For example, a typical car radio has 4 channels. Front left, Front right, Rear left, Rear right. When you get into custom applications and multiple speakers, then channels become important. The retrosound head unit is a 5 channel. Front L/R, Rear L/R, Subwoofer. You do not have to use all the channels.

I am assuming this is going into a 66 pickup? This will be a challenge... It sounds like you're keeping a dash mount radio. If so, then I would get a Retrosound head unit.

Next, install the dash speakers you already have. If you want more, we can go from there.

Here's where it can go 2 ways:
4 channels powered from the head unit. (typical factory car radio setup). 2 in the dash, and 2 in each rear corner.

Semi-custom; Subwoofer (bass), mid-range, tweeter. This will require one or more amps and the price goes up significantly. I was just looking at 66 dash pictures. There is no where to put a speaker. Your best bet is upholstered panels in the kick panel below the vent. Though speaker choice will be very limited, you will somewhat be able to aim them.

One more question.. What does your daily driver have? Simple radio and speakers, or a factory premium system?

Hope this helps. Like I said start with a head unit, and go from there...