Non-A-body 46re q’s

You can up the clutch count in the O/D section. I found info on that in one of the diesel forums, don't remember which. Google
To increase clutches within the 3 speed portion, you might need deeper drums, might not, do some research and just be aware. I can't offer much specifics, as I did the upgrade route on a 42rh. I think I got 9 clutches in the or section, no drum needed there. O/D is the same unit between all models of this series, just different clutch counts.
Between 4x2 and 4x4, the trans and Overdrive are the same, but the tail housing is shorter for a 4x4. If you are ordering a complete unit, then yes, You probably need to order a 4x4 unit. If you want to learn something, Rebuild it yourself If you've got the time and patience.
I kick myself for not buying an immaculate 5.9 I saw some years back.