I just acquired a 70 XKE roadster from the original owner and wanted to verify that it is the original engine. Where is the engine number located?

 

Not sure about the 1970 model but on my 1966, the engine number is clearly visible at two spots: on the backwards facing surface of the head just ahead of the forward-most spark plug; on the block surface adjacent to the oil filter adaptor (just shine a flashlight between the carbs towards the horizontal block surface).