The stock dash speakers are designed to drop in from the top of the dash. They have two "tabs" that support the speakers in the dash.
I installed 3.5 inch Infinity "Kappa's" in the dash of my hardtop GN (sold). It was tricky, but once I figured out how to make it work, it wasn't too much trouble and the finished installation was worth it.
In order to install the deeper Infinity Kappa speakers I suspended them *below* the mounting surface by using 1/4" long nylon spacers and longer screws to "hang" them lower than the stock mounting position. I purchased the nylon spacers and longer screws at a hardware store for less than $2. In this configuration, the speakers fit perfectly below the stock, unmodified dash cover.
I also used the same type of nylon spacers (but longer) to install stock size Infinity "Kappa" speakers in the doors. The stock door speakers have 3 "legs" or "stand-offs" that keep them from sitting too deep into the doors. I found nylon spacers the same length as the legs on the stock speakers. With these spacers, I was able to use the stock mounting screws and mounting holes.
I'd take pictures, but I sold the car a year ago.
HTH