After messing around with the adapter rings, moving the position of the speaker, and a lot of pushing on the 6.5 inch speakers, I took them back to Circuit City. I exchanged them for a pair of 5.25 inch speakers and VIOALA they fit perfectly.
Man wtf, you have the only 300ZX with a 5 1/4" speaker in the back, in the history of Nissan. If I had known, I have some badass PPI carbon fiber speakers I never even used in my 280ZX that are 5 1/4". Oh well.
The speaker mounts dont look custom or anything either.... They fit perfeclty in the wheel well covers like they were meant to be there.... the screws and bolts that hold that metal plate in have fasteners in them that are made to notch into the plastic of the wheel well cover... i swear it looks like the car was engineered that way.
I just put in 6.5 inch, 3 way Pioneer speakers rated at 230 watts. These are shallow speakers and I had no problem putting them in the doors and the rear. However, there are 6.5 deep dish speakers that won't fit in the rear of my 85 Z31. I had to be sure they were shallow. I was able to put the original covers over the new speakers.