oil in the antifreeze?

Ok looking for suggestions, the car I picked up a couple of months ago has started showing signs of oil in the antifreeze. Its a 64 Dart 270/ 225 slant with pushbutton automatic There is no signs of antifreeze in or on the oil dip stick and no steam coming out the tail pipe, The trans fluid look fine also, I want to say its a head gasket, but I am not hundred percent sure. The radiator gets full of this black / gray oily substance and it smells like trans fluid. I am going to pressurize the trans cooler in the radiator to check for leaks, then check the block with pressure. I dont care to replace the head gasket, just dont want to if its not the problem. I am use to headgasket problems showing up in the oil or out the tail pipe, not just as oily fluid in the radiator. I have driven it over two hours one way and it never got hot or overheated..