Somewhere along the line the trans issue got a little murky, early 80s I think.
My 85 D100 318 has a 904, and I am pretty sure it came there, based on how everything looks there.
Pre 80 I would agree on most everything having a 727, not sure about the \6, only 727 one I have ever seen was in a factory taxi car.
Pan shape will definitely tell you, lock- ups came later, looked same tho.
I did have a 92 Dakota 318 that from the factory had a 518 (overdrive 727), thought that odd, reg cab/short no trailer tow or other HD equipment.