Stock plugs used in the early days were AC-44 with .035 gap The AC44 has been discontinued but occasionally shows up on eBay.
Delco plug numbers de-code as follows: R=resistor type, 4=14 mm thread, next digit (4 or 5)=heat range (higher number is a hotter plug), S=extended tip. The R45S plugs are middle of the road heat range & should work fine if your engine is in good condition. Heat range is simply how fast heat is carried away from the plug. I believe the AC R45S (gap .040) is the correct modern plug for our “unmodified” 216 motors.
Harold
