[Chevelle-list] Spark plug question

[crobertson1]

I have a 396 engine in my 69 Chevelle with heads from a 73 Monte Carlo 454.  My problem is that the spark plugs seem to collect a lot of carbon causing the engine to run very rough after a short period of time.  The heads call for an AC R44T plug.  I was told to run a hotter plug so I went with an R45T.  I still have the same problem.  Any suggestions?  What is the hottest plug I can use?  Thanks   Craig