I don't belive it is too bad installing a new system in a 66 model. A lot of the guys in the club have left their stock system in the dash and installed a CD/MP3 player in the glove box and use a remote control to operate it. That way you dont have to cut up the car.
There a quite a few award winning designers in Melbourne for audio systems. In fact I read recently that Autobarn in Albury won a prize recently ! Bit far for you though !

Anyway JB Hi Fi in Brighton is worth a look as I have seen some good stuff come out of there. Other than that shop around and be sure to check out their previous custom jobs.
You could also come to the Mustang Concours in Port Melbourne on Sunday and speak to a few owners to get some ideas ???
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1995 Mustang GT - Vert
RHD Conversion, Vortech V2-SQ, 42# injectors, CAPA Chip, 18" BonSpeed Wheels, 3.73 Motive Gears, AutoMeter Gauge Pod, Pedders Springs, Koni Shocks, 13" Cobra Brakes, BBK Shorty Headers
~~ He Who Dies With The Most Toys Wins. But Is Nonetheless Dead ! ~~