Non A-body mopar question

I just looked at a 67 Coronet 440 that's low mile. Has 318 with torqueflite. I was doing some research online and a couple folks claimed their 67 or 66 coronet came with a 7 1/4" rear end. My question is what determined which cars got the weak rear end? One person stated that all 318 cars got the 8 3/4" but then a couple guys wjth 318 cars claimed.theirs also had 7 1/4" reads. Given that many here say the 7 1/4" was even too weak for the smaller dart with 273,Why would Chrysler put that weaker rear in the larger bodied coronet with 318?