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A/C R44XLS is slightly hotter than R43, R42,etc. The XLS part means they can be used in aluminum heads, as well as cast iron heads. Gap is standard at .035.
I think that the L-88 motor called for the R-42 plug or perhaps the R-43. You could experiment with both. I've tried the R-43 on my 427 L-36 and went back to the R-44XLS which is a good, all around plug.
I intend on trying the Rapid Fire plugs also. I think the "4" version would be a similar heat range to the 44. Also, I've read that they come standard with the new 502 crate motors.