340 rockers

Just going by the listing - they already don't know what they're talking about. If they indeed fit "All LA heads" then they should fit X and J.
However - if they are talking about heads using dual valve spring assemblies, they may need to be clearanced around the spring. That's fairly common. That being said, I don't think they are work spending the money on. Cheap rockers can lead to some expensive repairs fairly quickly if/when they fail.