Ok Guys, I need the straight up here.....I,ve heard a lot about which plug to use, I put AC Delco's in My 81 Turbo. I am looking to put either AC Delco R46SZ (2.50$) or Ngk XR45(3.75$) or AC Delco Rapid Fire(6.00$) in my 1977 400 6.6L. Some say there is no difference, others say there is..........Please give me your feedback, what you use in yours, what works best. Thanx Guys, Kanedit.