Cam bearings went out
When assembling a head one of the first things to do is install the cams with no seals or valves in it. Torque them to 8 foot pounds. They should spin very easily with no resistance. Any kind of binding means the caps are warped or the wrong ones. Over torqing even one time is enough to distort them. If the caps have been mushroomed at the head bolts or you see a small round burr in the threaded holes in the head that is a sign of damage.