Pretty much all the lower steering nuts are castellated, so you'll want some sort of retaining pin. I prefer to use stainless "Hairpins" myself - so I can reuse them comfortably if necessary (I never did like having to hammer a cotter straight and bend it again...)
Either way, I do strongly encourage the use of stainless for these pins, since they're exposed to all sorts of nastiness. Then again, I'm just cranky...
If you're not sure or if you think the nut has been replaced, check the threaded shaft end for a cross-bore. If it's there, get a castellated nut and PUT IT ON. As I recall, all the threads are "inch" sizes, so you shouldn't have too much trouble finding the right sizes for the job.
5-90