I've not seen one, although that does not mean they don't exist. I know, it does not reallyanswer your question.
However, if you are dead set on dual quads for a 351W, and can't locate a 2x4 manifold, there is a company that sells spacers allowing use of a narrower 289-302 manifold on the wider 351W. I inquired about a set approx one year ago and was quoted approx $250 +/-.
I did not purchase them however and have no knowledge about their quality, etc.