X-heads vs J-heads: what's the difference??

Years back I noticed that putting 1970 and up 340 intake on a 1968 69 340 with X heads. It was hard to get the bolts started. I first noticed this when trying to install a 70 Sixpack on a 68 motor. It seemed as if the X heads are shorter and the surface is cut at a different angle. Just things I can remember.