The Allison is reasonably large.
The manual transmission hangs down from center line a lot, while the Allison is bulky all around center line, and some more below.
Unless you really have to have an automatic I would stick (pun intended) with the manual.
Your indecision on what transmission to use tells me the trouble, work, and expense to swap to the Allison is not worth the effort.
Also consider how the transmission ataches to the engine.
What engine do you have, and what is the bolt pattern on it and on the transmission?
The Allison I'm guessing has an SAE #3 bell housing (round with 12 bolts), It would be easier if your engine already has the SAE adapter or flywheel housing on it. If you already do then still may need to find the right flex plate and torque converter too.
Grigg
