yeah, also if the ph16 doesnt work, then grab one from a standard ph8a (the normal v8 one that ford uses).
the oil cooler was designed to fit on the stock production block and thus had to seal just like the normal oil filters which is why you can use an oil filter gasket.
probably just goofed something up if I had to bet or like stinger said, you used one that is brand new and not been heat cycled yet so its a bit more stiff.