Unsure about the j10 258 but the older CJ 258 will mount right in. I can't imagine AMC making them much different. You will want to use your intake an exhaust manifolds though. The newer manifolds will be set up for elect choke, heater, ect. I believe you will have to lengthen your wire for the elect temp sending unit. On newer engines the sender is at the back of the head older ones its at the front.
Hope this helps.
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