I just rolled 365K in the 86 6.9.
These engines do not have a timing Chain or Timing Belt.
So I think that helps on the long life of these engines.
A broken one takes one out pretty fast.
No Valves to adjust.
They have mechanical fuel Injection, so nothing electrical to fail, except maybe the Fuel shut off thingy.
Not the fastest engine on the highway, but will always get you there.
You just bought a used truck, so you will have to take care of the previous owners lack of maintenance and abuse.
Goat