It's not that hard to make the cut out for a modern radio in the bezel. That was one of the first things I did. I think I just used a dremel with a cutting disk and initially cut the opening a little small. Then I used a grinder attachment and would grind away a little, test fit, grind away a little more, test fit, grind, test, grind, etc . . . And I don't recall having to do any grinding on the back, just making the hole the correct size.
As far as if you should do it or not (coincidently, I think that was my very first post on stangnet), it's your car. Do what you want. You're the one who has to live with it. If it makes you feel better, they're making more & more things available reproduction; eventually they'll come out with a repro radio panel for the console.
And for rear speakers, I just mounted some 6 x 9's in the package try and mounted some simple boxes to the underside of the package tray directly below them. Nothing fancy, but it's better than stock. I did have to drill a few holes through the metal part of the package tray for the screws so I could mount everything, but it's really not that big of a deal.
-Chelle