Small Oil Leak On a 383

Every time I drive it, there/s a small amount of oil build up, on the rear of the valley pan, on top of the motor. - I've checked the pressure sender, the valve covers, the breather & tube to the carb, -- Actually it would not be likely that those items could leak oil, onto the valley pan, any way. -- I'm wondering if the valley pan could have rusted, and caused a pin hole, under the insulating pad, that was (gone now) between the valley pan and the intake manifold. ----- Any ideas ?