Hi,
If you are planning on doing a total rebuild of your Mustang’s 3.8l V-6, there are several references to help you along. To answer your question, A Chiltons (Shop Manual) or a Ford specific reference for your vehicle are highly useful, SN is useful in all capacities..
Can stop there, but happy to help in greater detail if you’d like to add specifics to the post.
For instance..
1) Is this simply a high mileage project you wish to go through, or are there other reasons you wish to tackle this?
2) Are there any issues with the motor or transmission, drivetrain, electrical, or other areas, now? Noises, power loss, burning oil, poor performance, etc?
If your specific answer is above, all good. If you desire, please feel free to follow up & we’ll clarify things if you desire, again.. happy to help!
Best of luck, regardless!
Cheers!