Car Audio Questions - 64 Dart 270

I’ve got a 64 Dart 270 with cheap speakers and a cheaper aftermarket radio. Not quite up to snuff.

I’m planning out a new stereo system (If anyone has advice for amp, sub, rear speakers, kickpanels, etc. I’m all ears) and have a couple questions:

In the center of the dash (I don’t have a dash pad) there’s a square that has a bunch of little rectangular holes cut in it. Is that where a dash speaker is supposed to mount? If so, what bracket should be used, what size is it, etc.

I have the original AM radio from the car, as well as the faceplate. It was given to me by the previous owner. He took it out and cut the dash up a bit to fit a cheap Single DIN stereo. From what I can ascertain, I should be able to fit a retro sound radio in the faceplate and mount it up, has anyone done this or have advice? Or if anyone on this forum can rebuild the original radio with modern components I’d be happy to pay for time, resources, and shipping. It’d be great to have it back in the car.

What size sub would be good and where should it be placed. Under the back seat or the trunk?

Any help or advice is appreciated. I’m a novice when it comes to car stereos. Don’t even know where to begin.

Like cars, the budget determines how good it can sound. I use my smart phone as the source and wifi an online music service like Tidal. The lowest quality sound they put out is CD quality.

I do have a double DIN head unit to control everything, which feeds into a Zapco DSP-VI II processor then into 4 high-end Z-AP Zappco amps. One for the pair of Legatia X1 tweeters (A-pillars), 2 for the 4 Legatia X6 7.1" midrange/midbass drivers(doors) and one for two 12" subs under the rear seat in a custom tuned enclosure.

The DSP has time delay and actively crosses over all the speakers, so the bass sounds like there is a drum set in my dash. It's stunningly clear even at high volume.

At this level, it's all about component selection and installation quality. Good sound cost money, sir. How good do you want it to sound?