When I had my IRS I had 10.5 with Nitto 315s on the rear and IMO it set just perfect. The tire was kind of narrow (which nittos are) but other than that I didnt think it need a spacer at all!
10.5's you may not. They should fill in nicely.
New pics with 18" wheels:
Are you sure the stock rear springs in my 97 Cobra are progressive and not linear? They look linear.