Recently bought an r33 and have done much that
Doors not too much of a drama to take off, have 3 or 4 screws around the base, a small screw in the armrest handle well covered (normally) by a cap that flicks out, and the handle cover itself (flick out, slide toward the back of the door)
Then comes the fun part of easing the panel off. Helps if you pull out the armrest handle inset (the little cuppy thing) but its not entirely easy to do (or im doing it wrong) The armrest controls have a cable that clips in/out, unclip it and you're done.
Ive mounted in Sony 6"s with a tweeter up on the backplate to the side mirror. Once the door panel is off it just pops out. Had the best sound result for me, but play around with it once you have the main speaker in for best result. Everyone's head is in a different spot.
Hope it helps