are you saying you just have a motor laying around or is it in a bike?
If it is in a bike, than the model will tell us. fill us in.
You can't fit one for the other without mods
I know you can fit an "A" in a softail with adapters, don't know about the other way around.
The "B" motor is longer, so I don't see how you can get it into a setup built for an "A"