A couple of observations. As porky noted, on a straight truck the max legal length is 45' (straight truck and/or RV) so even if you use a cabover you will be well over legal length with that setup. I would think even one of those skinny little city truck type cabs would be over the 5' you have left for the cab, and I can't imagine putting many over the road miles in one of those. And you won't find a truck already long enough for something like that for the same reason. Definitely need a major frame stretch, and serious weight distribution calculations to determine where the axles need to be to balance and proper axle loads.
If you are thinking straight truck, why not just make it easy and buy a box truck like doc weaver? There are some really long box trucks out there generally for a reasonable price, and if you find one of the furniture mover type trucks you will have a nice big bunk over the cab already built in. Also taller than a shipping container giving you some headroom for cabinets, bunks, etc.
If you are thinking permanently mounting a shipping container on a custom trailer, just get a trailer already designed for a shipping container and just bolt the sucker on. No fuss, no muss. But even easier, if you are going to permanently mount the container to a trailer, why not just get a used semi trailer? They are definitely a dime a dozen so why go to all the hassle of building what is essentially a semi trailer when you can buy one cheaper and not have all the work? Most of them out there are 48' or 53', but there are plenty of oddball smaller lengths, drop decks, all sorts of things and sometimes those are cheaper because not every company is looking for that sort of thing.
I was looking at all kind of weird ideas before I fired up the welder on my truck, including using a standard box truck box, a furniture mover truck box, and looked at a lot of odd trailers on the internet. In my case for the scaled down project I ended up with it made more sense to build from scratch, but I did find out there is a huge variety of sizes and shapes of used truck boxes out there for really reasonable money if you shop around and aren't afraid to travel a little to go get it.