Take a look at this picture
. Retarded is to the left of the 0 (cam tower marks aligned) facing the car, as in this example.
You can just see (this is on a 560 engine) that the numbered markings are at 10 degree intervals, with a line for the intermediate 5 degree ones.
18 degrees out (yours) is an awful lot and I cannot see how the engine would run without damage unless the crank degree wheel has slipped. Is the 18 degree reading after installation of the new chain? If so, you probably skipped a tooth.
Are you sure you took the engine through a couple of crank rotations and that the cam tower mark on the driver's side was also aligned?
You are also changing the tenioner, I assume?