I have a 97 R33 S2 as well and i put a set of alpine type x splits in the front and back, both 6.5", The front speakers went straight in without modifying anything and i put the tweeters on the part that covers the wiring for the side mirror electrical wires, Definatly get sound deadening as skyline doors sound like absoloute shit without them, I am getting my doors done around august as well as my boot. I also have the crossovers in the same place as siddr20 With the wiring running back into the door, it looks good. the back i had a bit of trouble with, My car actually never came factory with parcel tray speakers (no idea why) and the people that had it in japan had a set of 6x9 pods that they took out before selling it (i could see the marks that it left) So i just took the factory parcel tray out and then made one myself out of MDF, just re covered it in carpet, and put the air vents in and cut out the speaker holes.. Was a bit dodgey getting the speakers in had to make some "modifications", And i just mounted the tweeters flush inside the rear side panel on each side in the interior, Right where your ears are if your a passenger , I have the crossovers mounted in the boot next to the amp.
(yes i am aware that it is on an angle, Its my first time at doing things myself haha)