scdascda,
The 43 is the heat range of the plug. My '62 327 uses AC R44's. I can't remember right off which way the numbers go for 'colder' or 'hotter' plugs but the term hotter or colder refers to how quickly heat can transfer from the tip of the plug to the body. Hotter plugs are used when idling a lot (i.e. delivery, taxi, or police) and colder plugs are used when the engine is worked hard (i.e. highway trucks or racing.) The back of the AC sparkplug catalog is a gold mine of information on the meaning of all the digits in the plug number.
I run an HEI in a '69 350 and run AC R44SX plugs. Resistor, 44 heat range SX = Extended tip and wider gap (.060) for the HEI.
