The 304 AMC is the IH 304 the same engine?

No, the IHC 304 and AMC 304 are two completely different animals, and as most say in the IHC cummunity, "the only similarity would be a handful of bolts".

International owned their own coal mines and steel mills and casted their own engines. Though their had been an AMC engine in a Scout (258), they generally used their own engines, with another exception being the Nissan SD33(T). FYI, somewhat on-topic; It has also been said that you can find some IH casting in MOPAR heads from the Vietnam era, where IHC may have helped MOPAR along during a time of steel shortage.