identifying what year a 4.6L came out of

Pictures or casting numbers?

Suggestion: Post the casting numbers of the heads or post pictures of the intake ports. The NPI intake ports are tear dropped shaped. PI are more square.

Post the casting number of the intake manifold. Post pictures of the ports.

Count the number of bolts on each valve cover. Count the number of crank bolts. The 1999-2000 Mustang should have a Windsor block. There should be a 8 bolt crank.

If high mileage, recommend that you inspect the timing chain tensioners before using the motor.

PI oil filler neck is on the passengers side.