I rebuilt a pair of Gran Sport hubs into a set of HP rims in the late 70s using 14/16 gauge spokes, and I drilled out each spoke hole with a very slightly larger drill bit, and slightly enlarged the countersink, as 'keithglos' suggested. I still have the hubs, and will be rebuilding them again sometime, with 2mm/1.8mm db spokes, for my Harrison rebuild (when I get around to it).