Hey guys---I helped a friend put a rack and pinion steering on a model A pickup last year. The rack mounts to the rear of the axle, and yes there is a splined coupler between the shaft going into the rack and the shaft coming down from the steering wheel. As the axle moves up and down, the splined shaft slides in and out of the coupler. It really don't move very much---a maximum total travel "in and out" of less than 3/16". The shaft coming out of the rack is tipped up about 10 degrees, and the shaft that leads up to it and the splined coupler is angled down at the front about the same amount, with the end closest to the car passing thru a support bearing., then two universal joints and intermediate shafts up to the steering wheel.