I think the decision should be between shawl DB vs peak SB + vest. If your tailor doesn't think he can make a good DB shawl for you, go with the peak SB. I think the peak lapels give the illusion of broader shoulders than notch or fishmouth.
P.S. if tall lanky Manton can look broad shouldered in his shawl lapel DB tux, perhaps you should ask Manton for his tailor.