MY uncle has made several of these for me over the years. He cuts a slit in the back of the manifold just behind the heat riser (like you were thinking of doing) cuts a chunk of steel, grinds it down to fit in the slot, then cuts a hole in the bottom of the manifold as close to the rear as possible and welds in a piece of two inch water pipe (very thick walled). There is one of these on my old '57 210 (I don't own the car anymore but, it's still in town). It's going on 20 years old!
