Problem while starting

I am having problems with my 2000 Chevrolet C3500, 7.4L truck.  When I start my truck after it has warmed up it tends to flood itself out when starting. I have to crank the motor longer than usual and when it starts I get black smoke out the tail pipe.  I have changed the Mass Air Flow sensor in the air tube between the air filter and the intake.  there are two sensors I have replaced.  1. Mass airflow sensor 2. Air temp sensor....When I start the engine in the morning I dont have this problem. The times that I have this problem is when I have been driving for a while in town (stop and go) running place to place,  this is when I have this problem. I would appreciate anyone's ideas on fixing this. I am going to change the spark plugs but I also need some advice on removing and replacing the distributor cap.  It looks like it is behind and underneath engine components on this vortec 7.4L.  I hope this chore is going to be easy....Thanks Andy