Autolite shows the 303 as the proper application for your truck. If the plugs are all covered with soot, your fuel mixture is probably too rich.
I don't know about the quality of Autolite plugs produced today. Any plug in these trucks should run 5 to 10 thousand miles before needing replaced. Myself, I run AC R45s. Traditionally, they have been the best plug for GM vehicles. But that's the main reason I use AC, tradition.
