You must need the heavy, heavy duty springs. Or do you need struts?
As I recall, the 1100 leaves the factory with the heavy duty struts already installed. This is due to the weight of the cab.
When a 900 needs the heavy duty struts for a plow, you're sold the ones that go on the 1100.
I'm not familiar with changes in the springs themselves.
I wonder if helper springs from the auto store would work?