I have them for my racing seats. I had a custom bracket made for them out of L bar steel. Then just drilled a hole in the middle for them, and attatche them to old seatbelt places on the outside, and a custom bracket on the brackets for the seats. If you do not have racing seats, then just say no to this idea.
You can attatch them off center like that but its not recommended.
On my drivers side I have a total custom setup with everything attatche to the brackets and floor boards so in some case I do race or something like that, Im totally strapped in.