Don't know how new 540 seats mount, but the sports seats from the '80's E24, E28 and E30 all mount to the floor. For an E9 coupe, you must remove the existing seat track mounts and reinforce the floor. I got a set of E30 sport seats from a friend after he wrecked his coupe. He had those seats reupholstered in a pattern which looked like the original rear seats. To install, he had removed the seat track mounts and welded some steel channel across the floor under where the new seats would mount. I'll eventually get around to doing the same.