Its funny Len... as soon as I saw your screen name... I knew right where you worked. I see that we have played in the same sand...
As with Len's comment... it is true that you
dont not have to narrow the differential housing. However, if you are going after the deep dish wheel offset look... there is no other way and this would be the most oppertune time to do so.
From a performance outlook... having the the wheel flange in the center of the rim does lend you a small strength increase in more than one singular component. But... its nothing really to write home about.
If you need further consultation from a local differential builder in your area... ProGear down in Kerny Mesa (just a click or two south of Mira Mar) would be a good place to pick up an education on this very subject.