[non-OP]

IIRC, there is no "HD" 231 in the Dodges, only "D" versions which are supposedly heavier duty with a wider chain, but they aren't "heavy duty", that was the 241.

 

[non-OP]

That is odd as there was never a 231D-HD listed in the Rams, which you would think would have that option.

But in general the internals are the same, its the input and output shaft sizing, tail housing differences (for mechanical speedo or not). HD usually means a 5 or 6 gear planetary vs the 3 gear standard. case patterns are the same AFAIK