Rear of car, the rear sills, where the dog leg section is. However 240z's often rust out in this area so you want to make sure your car hasn't been bogged up in this area.
I also use the front chassis rail, where the castor rods connect to the front rail. To place jack stands.
In terms of not denting or damaging the floors, you could use a piece of wood between the jack and the chassis. I've also used rags on the jack stands as a form of cushion or soften the hard edges of stands against the relatively soft metal on the chassis / car.
Don't jack from the rear of the chassis rail under the floor.
I also tend to use the front cross-member as a jacking point to lift the car at the front.
Come to think of it, I usually jack the front of the car up using the cross-member, rear of the car via the diff and I put jack stands under the front chassis rails where the castor rods connect and on the rear of the car you can use the sills or the rear of the chassis (not rear rails) where the rear cross-member is. I'll try and dig out some photos, but basically avoid the soft metal areas like floor pans etc.. Basically any box section / metal should be solid enough.