AT545 is the base trans, no lockup. Not terrible, but not ideal, especially for highway, as they will easily overheat at highway speed. I don't think I'd want to tempt the fates cruising at 70 mph with one. They are more or less meant for low-speed city and rural route buses, not interstate cruising.
And just because it can do 70 doesn't mean it's a good idea. I moved a bus for someone recently that the seller said could do 65... And it could. At the max governed speed of 2600 rpm. And some will say, 'But they wouldn't set the governor there if it couldn't take it'. But that's max safe RPM, doesn't mean it's the optimum sustained CRUISING RPM. Probably not a good idea to run at 2600 rpm for a 10-hour trip.
Also keep in mind that with most, the faster you go, the worse the fuel economy will be. Some buses can get 7-9 or better at 65-70. Others do much worse. Most diesels will be happiest at around 1800-2200 rpm, whatever speed that grants is entirely dependent on the transmission's top gear ratio, the rear axle gear ratio, and the tire size.
Certain 5.9 Cummins are known for what is known as the 'killer dowel pin'. This is an alignment pin to center the timing cover that is known to work loose and ends up in the timing gears, which can trash the engine. I'm not sure if this one is one of those.
Overall, I'm not going to say not to buy it, but I wouldn't buy it for highway driving -- 70 mph is likely to kill that transmission quick. And that's if it's not overspeeding the engine. Cummins are resilient and known for taking a bit more RPM, but if I were a betting man, I'd say this sucker is probably pushing 2500 at 70 mph, and I would never cruise at that RPM with a diesel. More RPM = more heat = more stress. I'd say best scenario would likely be about 58-62 at about 2100-2200 if the gearing allows it (would require axle gearing between 3.90:1 and 4.11:1 with 37 inch tires). The 4th gear ratio of the AT545 is 1.00:1 (output speed is engine speed, basically).
You really need to know three things...
Rear axle gear ratio
Optimum engine RPM vs Max RPM
That info, with an MPH / RPM calculator like this one
, should give you the safest and best cruising speed for this bus.