The good thing about these large trucks is that they are standardized. Which makes them very easy to modify to do what you want them to do. If you like the way it is now with a COE that's great. COE tractors have fallen out of favor and you can buy one for a song. Other than the junkman, collectors like me, and Russia there aren't many buyers for these rigs. I have a few pics of K100's to give you some ideas of where you can take your truck. JB Hunt used K100's up until the late 90's and there are a ton of them out there. Regular cabs, flat tops, high rise condos, you name it. You can still buy a full O/O spec K100 tractor with all the bells and whistles for less than $6,000. That would give you a great cab to mount on that chassis, spare engine, transmission, and thousands of small parts that would nickel and dime you to death. Check Truck Paper for some deals. Don't let the mileage scare you, it's normal to get 1,000,000 miles out of an over the road tractor. Anything with 600,000 to 800,000 miles will be a good buy.
Since you have a lot of wheelbase on that truck and want to use it as a tractor you can easily add a cab with a sleeper. Depending on the height of the chassis (I'm assuming you have a lot of ground clearance) I would go with a flat top sleeper. With a really tall truck you must plan your routes to avoid things like underpasses, tunnels, low power lines. If you went with a 72 inch flat top sleeper (JB Hunt used thousands of these and they're available dirt cheap) you wouldn't lose very much. Plenty of room to mount a 5th wheel, K-Boom, and a flatbed. Speaking of fifth wheel what are you planning on using? Fifth wheel technology has come a long way. There are many different options now. Things like power adjusting, load leveling, and even off road models with an extra pivot point to handle off-camber maneuvering.
Whatever you do with it I hope you enjoy it. You're very lucky to have such a versatile chassis. You can take it as far as you want to go. I wish I had something like that. I'd likely keep a COE cab on it, as that's a pretty good set up for getting the most out of any given wheelbase. For sure I'd add another set of powered axles. Making for 12 wheeldrive. Just for the wow factor if nothing else. Despite it's inherent impracticality I would add a high rise condo sleeper to it. Then have a full length dropside bed. With a crane of course. The truck would recieve a re-power. I happen to know of a 725 horsepower Caterpillar C-16 that needs a new chassis to live in. Back that up with an Eaton Road Ranger 18speed. Throw a killer paintjob on it. Something red, white, and blue. Have a mural on the sleeper. Then clean house at truck shows.