It's hard to get numbers unless you happen to have good records - remember the <92 trucks reset every 100K miles, and the 92-94 every 400K miles(399,999->300,000).
Also hard to determine if something has been replaced or not, unless you can see an obvious mark(like serial number being way out of whack, or an intake manifold with a way different year in it.
I have a good 5 trucks:
Tan '88 - Unknown miles(but probably in the 300s), motor needed to be rebuilt due to massive wear/rings gone.
Blue '88 - looks like original motor, again unknown miles... motor works fine.
White '87 Cab & Chassis - again,unknown mileage; saw reference to 200xxx in paperwork found; has a rebuilt motor in it(has a 7.3, not the 6.9 it originally came with).
Blue '92 - Speedo reads 3xx, has a rebuilt motor in it
Red '93 - Speedo reads 3xx KM(canadian truck); we know it's rolled at least twice, came with an '88 block rebuilt in '01... which needed head gaskets after the serp tensioner broke and the engine overheated a bit. Currently has a '93 IDIT motor in it that seems to be all original... again, unknown miles.
My take on things is that it's really hard to find mileage, or even worry about it. You look at the condition of the motor in the vehicle(mainly in terms of oil usage if it's not leaking, and possibly compression), and if it's good, she'll run for another long time as long as you keep oil in her.
Injectors and IP are a wear item and seem to always be worn if you are buying a low-price truck; most people think the motor's shot when you see blue and white smoke due to the IP timing being way off.
Also, IMHO, blow by is not a useful metric for anything. New motors produce a ton of blow by(large ring gaps), worn ones do as well, and it all goes down the intake and gets burned.
I do know of a couple of people with over 400K miles on their engines, people who bought the truck new and actually know it.
Also, while only tangentally related, a friend of mine has a 7.3 PSD with 500K miles on it. Motor still runs great. Trans(auto) has been replaced once or twice and needs it again real soon.