I have also realized 2 other things: 1) I would have to hack up the interior of my Firebird in order to get anything like a modern sound system (in addition to the $$$) and 2) NOTHING I could do with the Firebird is going to make it sound like the factory designed and installed Shaker 500 system in my late model Mustang.
You are absolutely correct on both accounts! And this is why purists hate me right now.
In my quest for tunes I completely destroyed two kick panels (original 1968...but scratched and buggered up from age anyway) and 2 door panels (reproduction circa 1998, no big loss/easy to replace), not to mention cutting some steel out of the kick panel area of my car and a big round circle out of each door shell (all of which can be put back in place by some fabricating and welding).
Hack up my interior I most certainly did! But it is all reversible, like everything is, with some time and money.
And your Mustang's sound system sounds MUCH better than my Firebird's does...no doubt!