All of the underpinnings will interchange as a bolt in with no modifications is my point.
Can you bolt an A body k member and suspension into an E body as a bolt in with no modifications?
but how many of us run C body brakes on our A bodies?
how many of us run a B body rear end ?
sure, some things interchange, some dont and some kinda sorta
the point is, who decided that a 81 cordoba is a J platform and 81 diplomat is an M ?
Ma Mopar did, so there must be enough of a difference there for them to go through the trouble of not making them the same
and to make clear what was "the same" and what wasnt, they put a code on em
now, it makes sense to make as much transferable between all the platforms to keep cost down