Ok, first off. When you note you've got an HT-70 ... everyone will tell you you're wrong ... you;ve got the wrong number, etc. You are NOT crazy, you DO have the right number. When I phoned Allison asking about my "HT-70" with my "8V71" in my "MCI, MC-8" ... they told me I was mistaken and was missing a number. After my repeated, "No, it's an HT-70." ... It took in an in-person, "Wow, no sh*t ... never saw that before." I had at least 3 or 4 mechanics from Allison looking at the bus. One took it for a drive with me in the buddy seat, checking it out. SO, no ... you got the right number.
That said, I would avoid using the HT-70 transmission. This mid-70's (evidentaly) Custom Coach spec'd transmission was most commonly found in off-road dump trucks, fire engines and the rigs which push the airplanes back from the airport terminals. Those vehicles aren't so much interested in a smooth shift.
This 6-speed auto is a hard shifting transmission until you learn its nuances ... and is NO longer supported by Allison. The Allison dealer suggested that "IF" I had transmission problems down to road, to swap it with a HT-740, or try and find a decent HT-70 in a junk yard for parts. They were kind enough hoever to provide certain important photocoped pages of the manual on it, as I could not find anything elsewhere on it.
The HT-740 is likely more common and user friendly for your application.