Yeah, the 6CTA is a mechanical 8.3. That one's built for 240 hp.
Don't be misled by the relatively low horsepower rating though. There are plenty of other engines out there rated at the same horsepower, but it isn't apples to apples. The 8.3 has a very long stroke and that peak horsepower comes at a lower rpm than other engines, which means gobs of torque.
The transmission is an Allison MT643. Good, strong transmission with a lockup torque converter, but no overdrive. It's a 4 speed with a direct top gear.
You can call Thomas with the VIN to get the rear axle ratio. Armed with that info, the direct top gear in the transmission, and the 11R22.5 tires, you can figure out what your highway cruising speed would be, and at what engine rpm.