MJ, did you remove the skin from your chicken pieces?
Yes you can substitute spelt flour for plain wheat flour. When baking with spelt flour you just need to be aware that it behaves a little differently to wheat. You need to either reduce the amount of liquid in the recipe by about 20% or add 20% extra flour to get the best results with spelt.
I found the lumps made it more authentic. KFC batter is quite thick and lumpy (from what I have been told of course ).