two thoughts here that I've been milling over:
Walter Thomas Ackerson - the three men confessed to beating and throwing him over the ledge when they were teens. Could these be false or coerced confessions? Possibly, probably not? However, one did say they weren't sure he was dead when they threw him over. Could he possibly have survived and took off, hitchhiking across the country? People have survived similar attacks. Again, probably not but this would explain why he was never identified, if he was thought to be dead all the way across the country.
Newburyport - I'm from the area. I've been really scratching my head as to why someone would choose this particular area to dump a body - just seems so bizarre to me. Newburyport is a very upper middle class suburb of Boston, and it's also one of the more populous towns in that area, making the Newburyport exits pretty busy at any time of day / night, with a strong sense of community safety. It was the same in the 90's. It just seems like the last place you'd do soemthing you didn't want witnesses to. There are stretches of the highway that are much more remote, and if you've made it to Newburyport, you're almost into New Hampshire where you could easily find more remote dumping sites. Even if remoteness is not a high priority, there are certainly more run down, less watched by police areas close by. Does this point to someone who doesn't know the area? Or does it point to someone who does know the area and feels comfortable there because of that?