Trans could also be a 741 which is the electronic version of the 740. It does have a allison box(ATEC). Hope that helps. Does sound like it is not shifting into 4th gear. Should happen at about 55 mph
You have a shift pad or shifter tower is the engine is electronic or manual there were a lot of the L-10 E that had the B400 behind it you could have a 741,746,747 ,748 or the B series if a electronic transmission
On the drivers side above the transmission oil pan about the middle of the transmission is a tag that will tell you what transmission is there keep your fingers crossed it doesn't say 741
OK there ya go Jim, Clifford & chessie4905 have told you pretty much what ya need to know.
Yes the "Allison box you found will be for the trans.
Now ya just have to figure out which Allison.
An yes I'm familiar with "Sodrel's" Free Enterprise & Star of America (and trucking co) operations.
Even if it was converted before they bought it, they should know what transmission it has. (they did service it didn't they? hard to buy the right filters, & parts without knowing what it is!)
BTW the Hound did still have some manual transmissions up into the mid '80's.
I know this because my uncle used to own a '84 "9" and a '85 "102A3" that both had been hounds and still had 4 spds in them.