I've built a few small block Fords, and they are easily capable of 100,000+ miles, if well maintained and not abused. While I'm not running a 408W in a Cobra, here are the things that might hurt that longevity, assuming a competent build:
1. Cooling. Improper cooling because of bad installation and poor airflow will kill any engine.
2. Rust. If the car sits for long periods of time, rust can form inside the engine.
3. Lubrication. If you start it and don't allow it to run at operating temp for say, 20 minutes, sludge caused by byproduct of combustion will build up in your
oil. This is also why a well-operating PCV system is necessary. It's also why I like running a hot thermostat. The thermostat allows the engine to get to temp quicker than running it open.
4. Abuse. These are race cars. If you race them, they will wear faster.
I won't run someone else's engine in my car. I don't think anyone else cares more that my engine is built correctly than myself.