I own both a 2003 636 and a 2005 z1000 (same as 03-06) and the seat is not the same from what i remember, i believe the rear tail cowl will bolt right us as well as the tail light but the under tail mud guard is different in the fact that the z1000 one needs the hole for the seat release key slot.
In the stable: "Studies show geniuses choose green"
2003 zx6r 636 Green 118whp (my baby)
Husaberg fe501 street legal 55hp
2005 Honda cbr600rr
2005 z1000 RawTI
2005 Yamaha FJR1300 Abs Blue