340 x or j heads

Rite next to the spark plugs cant miss it. If the heads are 587s they may or may not have a J by the spark plugs. I have a set of 587s off of a 72 340 that have a J cast on the underside but just what looks like II by the plugs. Why are you thinking only X or J? Could be O,U, or Zs as well depending on year and assembly plant (all 915 castings) only the early 340 came w/ the X (894 casting) 894s 68-70, 915s 70-72, 587s 72-73. Id start by checking next to the spark plugs.