Okay, I'm a little dumbfounded right now, as I'm just doing my first spark plug change since the engine swap.  What I pulled out were AC R46S plugs and yet upon picking up my new ones (that needed to be ordered) I was handed AC R43S plugs.  They're slightly taller, apparently cooler plugs as well, but could this be an appropriate match?  I apparently have a '69 400 installed now, from a Grand Prix.  Casting number 9790071, code XH with #62 stamped on the mid ports of the heads.  According to the shop owner, the AC R43S plugs are indeed for my engine.  Is this a replacement number to the previous AC R46S's?  I simply cannot find any info on this.  Help anyone?

Thanks,

Brian  

The major difference is the lower # plugs are colder. They will help an iron head/high compression motor run cooler. This should help possible detonation. The short answer is YES they are comparable. R=resistor,4=14mm thread size and 3or6 is the heat range and s is extended tip. Lower second digit means colder. If you have higher compression or lower octane fuel the colder will help. If they foul easily go back to the 46.
  I would go to another supplier if that is the issue.If your old plugs had no heat damage and you hear no ping then the 46 plugs may be fine.If the shop owner has your motor warranty he may be using the colder plug as insurance. This is not a bad thing if the 43 plugs burn clean.
                   Good Luck, Eddie