Well…maybe. Remember, these cars have only an odometer that reads up to 99,999.9 miles, then it's back to all zeroes. It's hard to prove an old vehicle reading "49648.2" or whatever doesn't actually have 149648.2 or 249648.2, etc.
Maybe the 50k miles is correct, and if that's the case and it is some valve-related failure, and the bottom end's in fine condition, then you stand better odds of getting away with just a head repair (it's the guys with high-miles engines who want to do just the bottom end or just the top end who find themselves having to do both anyhow).
But even if the 50k miles is correct, it's still reasonably possible this engine needs major work of one or more kinds. Prolonged under-use (i.e., short trips and long periods of sitting unused) causes its own kinds of deterioration and failure in engines and parts. And while the Slant-6 and these cars were sturdier and better-engineered than most of their competitors at the time, by today's standards every step of the manufacturing process was primitive and poorly managed, so the spread of durability in individual old cars and engines is much wider than in individual units of more recent or current car and engine designs. Cars just didn't last as long then as they do now. The average age of a car on American roads in 1979 was 5.7 years, and that was up from a lower figure in 1969. The 2016 average-age-of-car-on-the-road figure is 11.6 years, that is a little over double the 1979 figure. All of which is to say there were a lot more early failures back then—even among the generally more durable engine designs like the \6. If it's not failure due to usage or non-usage, it could easily be just plain old bad luck.
»yawn« Boring. Your car, of course, and my vote doesn't count, but…»yawn« Boring.
Now having said all that: What if it's just something easy like a faulty head gasket? I mean, you'd usually expect to see some other symptoms (violent radiator overboil, coolant smoke in the exhaust, two adjacent cylinders with similarly low compression readings) sometimes they fail less dramatically.
What'll it hurt to pull the head and take a look? It's not very difficult.
