the first gen Maxima came with a 3.7 R180.

Don't bolt anything to the floor, there is just not enough there to trust your life to it. Two bars across the seat area, tied to the door bars and then braced from the rear to the lateral bar on the main hoop is plenty strong and gives you belt attachement points that are all on the cage. You have to think of the worst case and decide who you want to go with, chassis or cage. And if your seat is on the floor and you belts are on the cage, or vice versa, you will be on the wrong end of a tug of war! My money is on the cage. If you are anywhere near Charlotte, NC, I can help you out. I have done 5 cars and they are all very safe and stable.

I can also hook you up with some good, cheap camber plates if you don't already have them, and you do need them.
Mike