I might not be much of help, but I pulled out a valve cover catalog, and a lot of them that aren't specialized have a caption beside them that say, "drill your own breather/filler cap openings". I have no experience Nailheads unfortunately, but I see some finned nailhead valvecovers on www.speedwaymotors.com;
maybe they have the original pcv valve. I would definitely recommend keeping the motor as original as possible, though. Hope I've been some help.