Problems setting transform.position.z in C# script.

I'm trying to set an empty GameObjects local Z to a distance set by a RayCast Collision. I seem to have everything working... but the correct method to set the Z value. Would love to know the correct way to do this, apparently position.Set isn't getting me there.

 using UnityEngine;
 using System.Collections;
 
 public class focalPtColDist : MonoBehaviour     {
     
     public float distToCol = 0.0f;
     public Vector3 posTemp;
     public Vector3 hitPosTemp;
     
     RaycastHit focalPtHit = new RaycastHit();
     Ray focalPtRay = new Ray();
 
     void Update() {
         
         focalPtRay = Camera.main.ViewportPointToRay(new Vector3(0.5f,0.5f,0.0f));
         posTemp = transform.position;
 
        // if (Physics.Raycast(myray, out myhit, 1000.0f) && Input.GetMouseButtonDown(0))
         if (Physics.Raycast (focalPtRay, out focalPtHit, 500.0f)) {
             print ("We Hit!");
             hitPosTemp = focalPtHit.transform.position;
             distToCol = Vector3.Distance(hitPosTemp, posTemp);
             transform.position.Set(0.0f,0.0f,distToCol);
             
         }
     }
 }