I got a whole shed full of ironhead Sportsters - and the best plugs to run are Autolites. They don't foul as easy, they're cheap, the tip is on for good - and they index easy. These are non-resistor plugs for points and magnetos . . .
Ironhead XLR Heads (long reach plug): Autolite 55
Ironhead Stock Heads: Autolite 4316
K, KH and KR Heads: Autolite 4316
(PS. Don't put the long reach in your stock heads or you know what happens)