If you insist on something different... I would look at IDI transmission pairings in heavier International trucks. This thread has some details...
That is good to know the 404/446 ih have a bell that can be used, that makes it one less rare part for going to sae trans. Founding this bell looks to be fairly easy compared to the one for the 6.9/7.3, @ISPKI these came up in a quick search
You can fit an SAE2 in our transmission tunnel without modification. I swapped my engine and with that am running a medium duty transmission with it. One item to consider is that single shaft overdrive transmissions are not really common. I did manage to find a FSO-6404A which is like a ZF-5 with an extra low gear. So far I really like it.
Another consideration is the clutch disengagement, the stock slave cylinder is not just going to bolt up without a custom mount.
Good to know there would not be much fit issues, I like the spread of the FS-6406N paired with 3.55 final and 35in tires. What did you do about the clutch hydraulics?