If it is an auto trans. it could be trans. fluid getting in from a cracked cooler in the radiator.
The early models of that engine had problems with the block cracking in the lifter gallery. That let oil get into the cooling system. It is hard to diagnose because it cant be seen without a complete teardown. There is no fix for it other then another engine. Keep up the diagnostic work, but if it continues I would be looking for a good 250 engine.
