I went the other way a 1940 Od (Dodge R7) into a 50 Plymouth so not sure how relevant it is. Also what version of OD as I thought they were not available until 1952. I ended up with a 1950 bell housing, a 55 230 and flywheel, and then the 40 OD. The shifter arms on the transmission on mine was the only change. The speedo cable worked just went over the top of the tranny. So I guess the best answer is that it will fit but you have to remember to swap enough parts sometimes and keep an eye on those you don't.
Also the wiring diagrams for both OD transmissions are available in the resource area. He should try to get the associated relays etc. if possible.
This may be a good time to look at clutch pressure plate etc. since it is that far apart. Its the domino theory after all.
With that in mind he may want to go with a switch mounted kick down like Blue Skies. He also needs to remember that with the OD engaged regardless of model It will not hold in gear going forward on a hill.
Sorry turned this into a laundry list almost.
Al