Good thought. I have over 30 years experience working for Douglas Aircraft and Boeing as a materials and process engineer. Aluminum has been used in primary aircraft structures for many decades and they experience many types of stresses. Different alloys have different physical and mechanical properties. It really is important to know to what temperature the aluminum would be exposed. The induced stress from having two different metals expanding at different rates (iron head vs aluminum manifold) would be mitigated to some degree by the exhaust gasket material and thickness. I believe that an aluminum exhaust manifold could be designed for the slant six application with enough knowledge of the operating temperature. Fabricating a few might be costly.
