DO NOT CUT THE PARCEL SHELF. It weakens the rear of the car, according to traffic regualtions. I fitted nice 6x9 pods/spacers in mine and havent touched the metal. Check pix below. All automotive stores should be able to help you out.
As for the head unit, most EA-ED falcs came out with factory Pioneer gear. The head unit has some brackets on the side of it that you will need to use again with your new unit. I bought a new Pioneer unit, and used these brackets, bought an adapter wiring loom so I didnt have to cut the original plug, and everything fits like factory. Two screws hold the front facia on at the top ( under the vents) then 2 (security) bolts are holding the stereo in. Get a Torx set for future use to get these out. I then used 2 more security screws to hold the facia in after installing the new system. Cant be too careful these days. Its quite easy, but if your not certain, get someone who is.