Manifold studs - which to use

The problem is galvanic corrosion which is when disimilar metals are put together and the transfer of electrons causes one of the metals to in effect, decompose. True, mild steel and aluminium will have a problem like this, but not as bad as aluminium and stainless steel. As an aside, some stainless steels are not as strong as high tensile steel and may snap under certain circumstances. Brass nuts would be better than copper. Assembling everything with a high temperature copper grease will help taking it apart again though it may smoke for a while!