Exhaust back fire

My 1969 chevy c 10 back fires when your driving at a steady cruising speed, its fine while accelerating or decelerating and has all its power but once you hit a steady constant speed it starts to back fire. I have changed all plugs, plug wires, distributer cap check for air leaks,
was told it might be my cam but then if it was the cam would it not back fire all the time as this is not a steady back fire. Motor was just rebuilt 7500 km's ago and just recently started doing this only thing that wasn't new on the rebuild was the holley 650 double pumper that was put on it.
Could it be the carb or the MSD ignition box.