Right, most would have J heads although it's possible it might have some with another letter. The "593" X-heads were being phased out in early 1970 and replaced with the "915" heads, some 593 heads made it onto 70 model year cars as well as a multitude of other letters cast into the same "915" heads; U,O,Z,J, etc. In fact some cars came with two different letters on the same engine, example one X head and one J head.

By the time 1971 production began it was a pretty safe bet that you would get "915" with the J heads.