In post-op RT for head and neck cancer with a close or positive margin, is there data suggesting that 66 Gy at 2 Gy per fraction provides better local control than 63 Gy at 1.8 Gy per fraction?  

I do not think there is any good data to support this. Historically, 2 randomized trials come to mind.  The first is the MDA Peters et al trial, that looked at varying doses with 1.8 gy fractions, and concluded that 63Gy was the optimal dose for higher risk patients.  The second study was...