A split neck is unlikely to have anything to do with headspace.
Watch your rifle as the bolt closes to final lockup. You will notice that the rear of the bolt is rising against the locking shoulder in the receiver. It is possible that a bit of wear has occurred, and that the bolt has dropped slightly. This could cause an off-center primer strike, and the slightly eccentric expansion you noted.
The only real test is to check the rifle with steel headspace gauges, preferably NO GO and FIELD. It is not unusual for a used rife to close on a NO GO. It should not close on a FIELD.
If the rifle does have excess headspace, the easiest solution is to handload cartridges sized to fit the rifle. This works very well.
