The gap is simple. The previouls owner probably got the dealer run-around and ended up finding a cheaper solution on the engine and tranny repairs ...just like a lot of owners do.
Like I say, you probably shouldn't buy it. The right owner for that tk is one who is willing to assume risk. If all those items listed above have been ironed out (new engine, new/rebuilt trans), then it'll be a sweet ride for someone. But maybe they weren't. Who can say? Any truck/car can go bonkers at any time. But if you're not willing to assume the risk. Don't even look at it. There are other trucks out there. Don't buy something you're not the most remotely comfortable with. Now that someone put a bug in your ear, everytime there's a minor squeak in the glovebox you're going to think you bought a lemon and kick yourself. That's how most buyers are. LOL.
It may be that a long powertrain warranty may help reduce your risk. But the fact is, most people can't handle the idea that their sweet, cherrished, beloved truck might in fact break down at any possible moment. And it's entirely possible with any truck with that many miles. That's life.
I've seen so many times, co-workers or other people I know dump a whole lot of money into their truck ...fix all the expensive things that are wrong. And what do they turn around and do? They sell it or trade it off. My neighbor had a King Ranch truck. He literally spent $16K fixing the engine, suspension, brakes and other things on it. Brand new expensive tires. I know him well. I mean, he had many things on the driveline repaired that didn't really even need it (shop suggested it). It was a fairly new truck too. Then he literally trades it off for a brand new F350 less than 2 months after he got it back. The new owner (guy in our home town) ended up with a darn nice truck with tons of new driveline stuff on it and rebuilt diesel engine. He got a heck of a deal, he's tickled. I see him drive the ole king ranch around town now and then. Sure it has a few minor boogers like a small rust hole in the bed, carpet stains ...minor things. Maybe he had those things fixed. A lot of owners do that it seems -- dump a lot in a vehicle, then sell it. It must be human nature. Probably they just got done writing that 'last' check to get it fixed and reached their pain threshold. IDK