I thought the B150s with 3.9 got the 42

AFAIK the ZJ with 318 was 42

I admit I never thought about a 42 in a Ram

It's not without precedent to put a weak trans behind a baby V8 but not the larger V8 option. Ford refused to use the AOD or M5OD behind the 351 but did place them with the 302

 

its not that this isnt the case, I just dont see that much difference between the two power plants, I mean yea the 5.9 has higher specs, no argument there, I guess I've never had the privilege of daily driving a 5.2

40 cubic inches doesnt seem like much

 

I think it's more for strength propelling the vehicle + load. The diesels are the same displacement as 5.9 gasser and they get the 47RE. Will it work, Yes but for how long? The factory figured out what trans would fit the vehicle and what it would be used for. I remember a company Furd work truck, The trans was rebuilt 7 times before I stopped counting. Don't know if this was Furd's fault for putting a weaker trans in than it needed or the driver. Maybe a combination of the two because the driver was not known for babying vehicles. If it were mine I would get a 46RE rather than have to do the job twice.