You just made me think of something I forgot, which is very important regarding making a 24' box work with a 120" BBC and 70 inch sleeper. My 24' Morgan GVSD box was either modified at the rear frame when my truck was built, or was special ordered from Morgan to be a little shorter. I have the inside floor length measurement at 23' 5", and there only about a 1/2 inch lip outside the door to the end of the truck. Morgan lists the 24' GVSD to be 24' 5 5/8" outside, so that 5 5/8 inches plus a little more was removed from mine. A apologize for not remembering this, and I can see it makes a big difference when the numbers are added up. Glad I caught you before I left.
But, not all box makers do things the same, and you might still be able to find a stock 24' box to work without changing it. If you know the exact outside length of your tractor and box before you do the build, I think that is the only way to be 100% safe you'll stay under 40'. The 120" bumper to back of cab spec many of the makers use is not always exactly that either- on my Columbia the 120" + 70" sleeper should equal 190", but it's more like 185", or right on there with the 15' 5" of the International you are looking at.
So I guess you have one of two options if you want to use that tractor with a stock used 24' box that actually come in around 24.5'.- either have the box shortened up a little (probably still cheaper and better than frame mounted tool boxes), or use a 22' box.
To keep things simple and cheap, you look to be safest with the 22 footer plan. The person who originally built my truck obviously didn't mind messing around a little to get the extra box length, I wouldn't do it myself.
Taking a wild guess, I would say the box I have was special ordered, as the fleet owner from NY who built these (there were two idential trucks built) special ordered the chassis from Freightliner with the long double frames- they are not stretched tractors. He spared no expense on the builds, I estimate these came in at around $90k+ each. They were leased to Panther II. Mine was repoed after 54k miles, the other one I saw about 6 months ago being towed to an auction yard near Harrisburg, PA, still dressed in its Panther uniform.
-Weave-