I tried to scribble into the ROM picture to be clearer.
Hope this helps. I don’t think that any panel can shrink or warp sufficiently not to be mountable any more. Maybe you start - just for an attempt - finding the screw holes in the outer metal carrier and aligning them with the outer edge of the underscuttle casing. Then it is quite clear where the trailing edge at the rear side belongs behind the knee panel.
Then you just do it the other way round and begin by manoevring the underscuttle casing around the handbrake mechanism and just “hinge” (for lack of any better expression) the inner edge of the casing above the upper edge of the center console cheek panel. Then guide the rear edge left and right of the steering column behind the knee pads. Finally push the outer edge upwards until you find your way to the screw holes in the metal carrier. Before you fix these screws you first have to put in place the upper quarter casing until the screw holes of the underscuttle casing, of the quarter casing and the metal carrier align. Now put in the screws.
75 XJ6L 4.2 auto (UK spec)