The stock lugs are threaded 1/2"-20, and a 9/16" UNF should come back to 18tpi (9/16"-18UNF should be the callout. Just for your edification...)
Are they "acorn" style lugs? Does the centre hole fit well over the hub? The "acorn" lug fits into a tapered seat, which also helps to align the wheel - and if the centre hole fits closely over the hub projection, that will also help to locate the wheel. If those two work, you should be fine. I'm not sure if there are "lug washers" out there to help expand the taper of the lug nut on acorn lugs - and if you were to install larger (9/16") lugs, you'd have to take the shafts out and the hub bearings off up front, and have them bored out to accept the larger studs. Doable, but it could get spendy (especially since those holes are for a press-fit splined stud, meaning that the tolerance on them is fairly close.)