The DT360 is an inline 6 diesel. It is just the DT466's smaller brother and is considered equally stout.
If you ask around you will quickly find that what limits the speed of our buses is not the power, but rather the gearing and speed governor on the engine. I know mine could do better than 62 mph, but with that governor starting to cut fuel at 2500 rpm and fully stopping the juice at about 2600....well...I'm just kinda stuck, 20 mile downgrade or not.
As for the AT545....you will often hear the bad about them and many of the complaints are legit. However, this tranny was used FOREVER and in all kinds of applications. I would be willing to go out on a limb here and say 90% of us with autos have an AT545. Another 5% probably have an MT643. The rest are people like MacGyver with weird stuff like a 730
the torque converter never locks up which means lower fuel economy due to slippage
not the strongest tranny out there
cheap to replace (cheaper than the beloved TH-350 Chevy tranny)
the torque converter never locks up which means torque multiplication at all speeds (ok...going out on a limb with that one)
there's not a mechanic out there that hasn't seen one
Personally...I know my AT545 emasculates my engine and cuts top speed, but that doesn't matter to me. It works great, shifts strong, and makes a cool whining noise from the gears on deceleration (I remember that from childhood on the buses too). Sure it has its shortcomings, but so do I and people still keep me around.
BTW...the MT643 or MT653 are the trannies you would have behind a DT466 in a perfect world as they are stronger than an AT545, have a lockup convertor, and in the case of the MT653, an extra gear.