As I look around at re-upholstery kits, I see two different types, one with recline option or one without:
https://jdm-car-parts.com/collections/interior-trim-nissan-datsun-fairlady-z-240z-260z-280z-280zx/products/datsun-240z-1969-1971-seat-cover-set?variant=1045889564
My z is a series one 12/70. However, the seats do recline, albeit only a few inches.