I looked in the bentley manual. It lists the following:
1992-1995 M42 318i/is/iC - Getrag S5D 200G/S5D 250G
(S5D 200G was discontinued 9/92 - also 200G & 250G are interchangable)
1996-1998 M42 318i/is/iC - Getrag S5D 250G
1992-1995 M50 325i/is/iC - Getrag S5D 250G
So basically the 318is uses the same g/box as the 325 after 9/92 however you can use a 200G in a 325 and vice versa.
Hope this helps, Nick