I have been running the manifold shown in Stock49's picture for about ten years on my 261. You are right it was made for a 216. I was able to bore out the ports to accept the rings. Not a lot of extra material there but enough. I have never run the water heat on mine. While the warm up might be a little slow I have never had any problems with it getting too cold. I suspect the wide flat bottom of the mainfold might pick up some radiant heat from the exhaust manifold.
I run the Holly/Ford 94s which are a 15/16" throttle bore and they work fine. I tried some later Ford 1-1/16" throttle bore carbs and the throttle response was terrible.
Gael
