Well, maybe for #3. But SAE does not necessarily mean medium duty transmissions. The Warner T19(commonly found in 1/2, 3/4 and 1-ton apps) was mounted in diesel Scouts using SAE #4 parts.the SAE are all med duty trans ( Allison ,Spicer ,rockwell ,fuller )....
I have an SAE #4 clutch housing that came on an NP4530. So with a simple adapter(see below) I could use this #4 clutch housing to mount a light duty NP trans(like NP435) to an SAE #3 flywheel housing.
This company makes adapters to go from one SAE # to another.
Note part numbers 26-01305 and 26-0035, both of which adapt an SAE #4 clutch housing to SAE #3 flywheel housing(what's on the 4BT you're considering).
The Ford/Mazda trans is not SAE so would not mount up as things are. But because of the Ford based 4BT/T19 equipped stepvans this trans could be used with available parts, IIRC. I'm sure I've read about that trans on this site.
Keep in mind you could definitely sell the SAE parts off that engine(flywheel, flywheel housing, starter).