rocker design can't dictate engine type, because the magnum uses a rocker design that is actually interchangable with several ford and chevy engines. As in directly swappable. Does that mean the v10 is a chevy small block?
If you replace every part of an engine with something new and different, is it still the same engine?
In all fairness, it doesn't really matter and has no bearing on anything. The OP is concerned about reliability.
Here is the way that generally tend to work. Turds die hard and fast or are repaired quite quickly, often with an updated part or more attention to assembly. The truck is 13 years old and has had plenty of time for defective parts to rear their head. For example many v10s had crankshaft problems early on. Suffice it to say, practically zero would still be on the road this late in the game with a bad crankshaft, most were replaced under warranty early on. A select few parts may have a slightly higher failure rate, like injectors for example (on any engine). but the cost and expected lifetime from that is not even worth worrying about or factoring into the decision. Just like I wouldn't turn down a magnum engine just because the valley pan may or may not go bad some day. Bottom line, any engine running good today, will prove to be fairly reliable overall, expected mileage based wear excluded.