43-5 was a commonly used plug, but hard to find; however (I found this after we talked, Ted): 44-FF was the plug listed for later 261s, and the C42-1 plug was recommended in 1962.
Gene Schneider also posted that the 44 plug was used in later 235/261 engines and the 44s are hard to find. He indicated that the 45s were the common replacement plug. Gene also suggested to keep away from the long version of the plugs - he had seen evidence that under certain loads (driving conditions) piston damage might occur.
http://www.sparkplug-crossreference.com/convert/AC_DELCO/C42-1
