Don't feed the trolls.
I'd say that you won't see much by way of big audio builds in antique/classic cars as most folks who restore them try to keep everything original.
If your handy with fiberglass custom fabrication I'd consider keeping your stock stuff stock elsewhere and building up kickpanels for component speakers. Kickpanels are a top choice location for good imaging and if you have room would be about the best way to not have to heavily modify valuable original parts and panels.
Been a while since I've seen one of those early Mustangs but I imagine they only have a slot for a 2 knobber head unit. I've seen some advertized as high quality to fit those holes but I don't think we've had any reviews here on them. A lot of folks will build something custom to mount standard single DIN or you could probably even bluetooth and mount a tablet somewhere. Again, how much do you want to modify original parts for the sake of that, and what's your level of skill?
Really what's your goal here? Good sound? Showy looks? Make your ears bleed?
Good advice about the trolls...
So what I'm after is good loud sound, with enough bass that you can feel but doesn't rattle the entire car apart. The only thing I'd consider is speaker pod kick panels. That said, I'm putting in the Morel Nano Carbon for that area (which will not need a pod enclosure), with the mid/high components grouped into a custom panel under the dash grill where the original 4x10 went (yes, they do all fit).
I'm going a little different with the best Kicker 4x6's as they are a direct fit for the original door speakers (that was a dealer option).
Head unit I went re-engineered original with the Classic Car Audio AM/FM/BT head unit so as not to cut the metal... It looks practically identical with the original except when powered on (again, not worried about that). That's about all I can get in the front.
Morel's run 100W/ch and Kicker's 75W/ch, keeping both at 4ohms. I've got an EQL for tonal adjustment, an EC20B for a crossover for the sub(s) which is where I'm stuck. That and the amps. I have a RF R600X5 (50W/ch @ 4ohms + 300 @ 2 ohms), however not sure if I want to go higher for the front speakers specs. I'm also not opposed to higher power JL amps either. Likewise, haven't picked the subs yet... however they will be firing forward. I'm undecided about the rear deck, if I want to pack 6x9's underneath and get another amp, crossover, etc. Electrical for this has been upgraded, even if I pack on another 800W or so. Likewise, doors, floors, firewall, and wheel wells outside and the entire trunk will be dynamatted. Also boom mat under the headliner. Going with a windshield antenna and removing the original one.
This is going to be a fair weather weekend cruiser (and cruise-in), not a show car.
So yeah, that's about it.... oh, and I've never done fiberglass... so would use the aftermarket kick panel pods IF they were sturdier... which is partially why I went with the thin Morel's. I also heard them in a friend's '61 F100 and they sounded good and they fit without cutting any metal.
