When you buy knuckles buy a matched pair from a disc brake axle. You want a matched pair because the tierod on non-flattop knuckles mounts from the bottom and tierods for flattops mount from the top. Therefore if you one swap one knuckle and you want lowsteer crossover (which I recommend) your tierod won't bolt in.
Even if you deside to do highsteer I still recommend getting a matched pair. Swap new balljoints in the new pair now and when you do crossover you don't have to try and replace balljoints then. Also if you run histeer and break some steering arm bolts you could install a stock tierod to help get you home.
Harley