... in removing the bang fitting I discovered that the previous owner had already done this to the 5 bolts on the starboard side of the vang fitting (going from 1/4 to 5/16). It appears they held it tight to the mast. As you say it needs to be pressed tight to the mast when threading the mast part or you could introduce a gap.

I will be interested to hear from others, though. I don't have to do it that way (have to say I do like the idea of having the fitting tapped so that it provides a steady guide when doing the mast)