[Freddo]

Warrens69gmc: Thank you so much for your reply.

I think that the hole for the plugs in the heads must be tapered. I couldn't get the R45S plugs to even start in the threads of my head. The shank on the plug just above the threads is quite a bit larger than the R45TS plugs that are presently in the engine.
I included a picture of the front of one of the heads and included them with this post. (Not thinking it will help much, but thought I would include it.) I don't believe that the heads are different from each other. Unfortunately, I don't have the casting numbers. Not sure where they are at on these heads, but it didn't look like I could get them without removing the head.
Thanks again for your reply. I appreciate your help.