240Z and 260Z steering racks are different. The changeover date is August 1973. 240Z racks will have a clamp-on snubber on the main tube. 240Z rack has circular input for pinion with press-in oil seal, 260Z rack has two bolt retainer at pinion input. 240Z rack has narrower mounting points and therefore the mounting bushes are different (thin side wall 240Z, thick side wall 260Z). A 260Z 2+2 rack is different to a 260Z 2 seater - UK lock to lock 2.6 turns 2 seater, 3.1 turns 2+2 (except for rest of Europe where they are the same) and 15.8:1 ratio 2 seater vs. 18:1 ratio 2+2! A 240Z is 2.7 turns lock to to lock and 17.8:1 ratio.
However, both use the same track rod ends, gaiters and inner track rod ends and the engine cross-member is the same (supercedes with upwards compatibility), so the rack will physically fit with the appropriate bushes. 260Z racks may have less mileage on them, but all Z racks are coming to the end of their useful life now! From the figures above you might regard a 2 seater 260Z rack as an upgrade in spec. compared to the 240Z original.