I saw a wierd situation once. A 1970 280SE had a 1966 250SE long block, and it kept having head gasket issues until someone checked the engine id number and realized they were using the wrong headgasket for the motor that was in the car. Make sure the headgasket you used belongs to the engine in your car, not the model and year of your car. I once bought a 1977 Alfa Romeo and had the same problem until I figured out it had a 1966 Alfa motor(they all look the same). Good luck.