Random update:
My late 260z basically can't have these seats mounted properly, centered to the steering wheel, without 2 things: bashing in the transmission tunnel, and fabricating new mounts. The stock sliders can be attached to the seat, but there's not enough room to move the main mounting bolt holes far enough to get the seat centered... it's just not possible IMO. I've ordered a Sparco mount and will update progress once it gets here, for future reference.