How to tell engine health un truck with unkown history

That’s interesting the average OH is at 660k miles. Doing the math, assuming it was done once at that mileage. It’s due again. No wonder your a bit nervous about it. Here We tend to OH around 1 m. miles. Unless a head cracks, or gasket starts leaking, making pressure in the coolant. Otherwise it’s watch for dropping oil pressure. Signaling worn bearings. Or excessive oil consumption, signaling worn rings, along with excessive blow by. Regular oil samples might catch an early problem of mixing fluids, or sudden increase of metals.I like Boxcarkidds idea of changing the oil, when it drops a gallon. Assuming that’s at 12k or even 10k miles. As the engine ages, it will need 1 gallon, then be 1 gallon low again, at 10k- 12k. Burning 2 total. Between changes. I’ve often ran mine (Detroit’s, and Cat)down to 2 gallons low, not adding any. And change it. As they seemed to burn the first one quicker. Just like an old gasoline engine. ( Gasoline engine’s consumption most likely caused by valve stem seals). None of my theories are scientifically proven, Lol. So attempt at your own risk. Best to add oil as needed. Not worth taking a chance. Also adding fresh oil to a old engine and old oil, can only help prolong the engine. Either way, you should be able to tell when an inframe is needed, before it “Blows”. My definition of “Blows” is when a rod goes through the block. The other stuff, can be done before symptoms show up, as PM maintenance. Or when needed, after the symptoms show up. Either way, the OH will be the same. New kit, head, injectors, oil pump kit. and rockers if needed. Plus any extras you want, like vibration damper, water pump, anything needed to protect the work. Sounds like you got a keeper. I’d just run it, change oil, watch for increased blow-by, low oil pressure, and if it has good power and fuel economy. Leave it alone. Get some use from it. Eventually it will need an OH. You’ll know when it starts using oil, or losing coolant, or oil pressure.