2005 5.9 Ram build.

Kinda like your hand there, if it ain't broke, don't fix it

380k miles is substantial for most engine, but a diesel?

Not unheard off

Plus, if the engine has 380k on it, I'm guessing 80% of the rest of the truck does too, so if you rebuild the engine, are you obliging yourself to replace everything else on it in the near future?