what 4 speed auto transmissions can i use? 46rh etc

There is something I just don't get with this trans fitment. I know the a500 can fit with just cross member mods where the 518 needs floor mods. But looking at the pics above the front of the 518 is the same size as the 727. Looks identical. I know the 727 will fit so it must be the overdrive units that doesn't fit. However the overdrive unit on the 500 is identical to the 518 so in theory they should need the exact same mods. But it's well documented in posts that this is not the case. I don't get it