Structures in caves, need help.

I have a 1,4,1 spike structure that I want to spawn in my caves of my custom dimension ( I do have the probability set to 1,000,000 and the grouping set to 16,16). I can't seem to make this happen because I think the "Spawn Location" is restricting that from happening. The "Underground" setting did work but not in the right way. The "Ground" setting I believe is for the surface only and the "Air" setting I think has a height restriction as well. I need the spike to spawn on-top of my specific block not "in" that block. Does anyone know how to do a work around? Mind you I am no good at coding what so ever lol. Thanks in advance :)