As most everyone has mentioned, the under hood stuff is most likely to be degraded. If you intend using Alloy Metal Products, then call NPD and see what harnesses you would need under the hood for your model and engine, then decide which you want to do, and the order to do them. Since they plug into one another, you could do them one at a time.
For my Mach with a 351w there is; Wire assembly engine gauge feed, alternator feed, wiper motor repair harness, engine ground, wire & plug headlight extension, wire loom firewall to headlight (this is the main under hood harness), wiring harness turn indicator (for the hood scoop signals). I believe there are four others, but they are inside the cabin or trunk.