If you're sure it's not leaking past the valve guides or valve seals it could be the intake gasket not sealing at the bottom. These will leak just enough at high vacuum to cause your problem. To eliminate the PCV as a problem just unhook it and block the port. Make sure the valve cover has a baffle under the PCV valve. One other culprit could be the piston rings failed to seat. When you first started the new engine was there any kind of carb problem that may have washed the rings?