the difference heads small block

X heads came with 2.02 and J heads for 70-71 340s had 2.02 valves ...all other J heads had 1.88 intake..
not sure about 1970...those could have been x or j but they were 2.02...

71 360s had J heads but they were 1.88

They changed sometime mid-year in 1970 from X to J heads...