Impeller Placement within Blower Housing

[youcancallmeAL]

I am trying to clean up our blower product-line drawings and I would like to provide an assembly dimension from the inside of the blower housing to the impeller. I would like to dimension the parts such that I have the greatest amount of mating surface between the impeller and the motor shaft. But this will move the impeller from the center of the housing. Can anyone tell me what would this do to pressure/flow/turbulence?

Second, can anyone recommend a site/book/class regarding blower design?

Thanks!