For '70 models Xs were still common for 340s. All Xs had 2.02s- not the same as Js. Js were used on some '70 340s, on '71 340s and 360s- but with 2.02s in the 340s. '72 all Js were 1.88 intake. Technically the heads for '73 were not J heads- typically they have AIR holes that were blocked stock but the design's different than later heads. After June, a motor is most likely the next model year. Honestly Xs are a bit better, but with the big price difference your money's better spent in improving something else or stepping above stock. Otherwise- the 340/360 heads are all about the same- take your pick of what you can find, a more desirable casting cheaper means worth more when it's time to go. Some of the late '70s heads can be hard to give away machined- great when you're buying, awful when selling.