Personally id go original if you can afford them. Better quality and fit but they are much more expensive than the aftermarket ones.
I have a set of original mercury stainless steel risers for a small block v8 if you are interested. quite rare and very good nick. They obviously don't corrode like cheap cast ones. One has a snapped stud on the side (I think they are used to mount ignition component / cable assembly) but that is easily sorted.
PM me if interested.