I have a full size MII console in my 78 Sedan. I didn't have any issues with it rolling to one side, but I did have issues with it being straight and level with the rest of the interior. For me, the important part of it was to have it down against the carpet, and flat/level, in the front passenger area. Mine sits about 1.5" above the carpet at the back end of the console, and I used some extra carpet and cardboard to make a "filler" section of carpet to fill in the gap between the carpet and the console. No one has noticed it yet.
Be sure you have the front bracket, and the rear one. The front bracket mounts under the dash at the front edge of the console, the rear bracket goes behind the e-brake handle, and the console is secured to the rear bracket at the rear most holes inside the arm rest compartment.