[David]

I would advise against using stainless studs on the aluminum case (and stainless for any exhaust application). The aluminum and stainless steel become a ganvanic couple with the aluminum being the anode and the stainless being the cathode. Since there is high potential difference between these two metals, the aluminum will corrode at a much faster rate than if mild steel is used.

Using austinetic stainless steel like a 309 or 316 is not recommended for fasteners especially on high temp applications. It's high temperature strength is not good and it galls easily. A martensitic stainless steel like 403 or 422 is a good high temp bolting material, but it's not readily available. FYI, martensitic stainless steel is magnetic.