small bolt pattern center bore?

hey guys

im looking at buying a set of wheels for the front of my 71 duster
it has the small bolt pattern, 9 inch drum hubs and scarebird brake conversion

i figured 15X5 would be plenty, they come with 2.5 inches of backspacing, should be good right?

anyway, US indy no longer stocks them, but they can drill them to size, which makes me want to know, what size should the center bore be?