This should be pretty straight forward if I understand you correctly.
you can get each nodes position, maybe just keep track of the last node clicked
var lastNode:SCNode? = nil
let currentNode:SCNode = placeNode()
currentNode.position = SCNVector3dMake(currentNode.position.x,lastNode.position.x+30,currentNode.position.z)
lastNode = currentNode
Would you please explain the code a little bit? I am working in unity and C# and my Swift knowledge is rather limited. Thank you very much for your help.