I own a 1978 El Camino. When starting, cranking is slow as if battery is weak. I have changed to a new battery but still the same results. Can someone assist me?

 

More than likley it is the starter solenoid. There is a copper disk that when switched contacts it, these get burned over time. I used to take them off, and apart , and sand them smooth. Your's may be new enough as to have rivits instead od screws that hold the cover on.