I think his question is more of a "How do i spawn them all lined up and beautiful" rather than "what are the farmable items".
The XMLSpawner just does random distance within the range. The only way i can think of...is to actually create 1 XMLSpawner per item with no range and place it manually exactly where you want it.
You would want to do it in a file so that it loads with createworld.
Just place one with location of (x, y). Copy paste that field a few times and change the y to -1 each time and it will create an appearance of line.
---------------------i (x, y)
--------------i (x,y-1)
-------i (x, y-2)
i (x, y-3)