Tim, this is what was available for 6 cylinder Mopar in Canada land for 1946-48 Chrysler built 6 cylinder cars, they should bolt into the diff housing on a stock 1946-48 Plymouth.other diff ratios in the Chrysler line up were , 3.91, 3.36, 3.58, 4.56, not sure if those were for 8 cylinder cars or not, but are listed in the 1940-48 Chrsler shop manual.
1946 Chryslers and Desotos used all of these ratios depending on wheel base, or if the car was equipped with fluid drive and 3 spd trans, or fluid drive and m5 semi-auto 4 spd trans.
P15s/D 25s in Canada quite often were equipped with either 3.73, 3.90,or 4.10, depending on geographic location and customer needs.
My Chrysler had a 4.3 diff, it was froma 1948 Chrysler 7 passenger car, I pulled a diff from a 1947 Chrysler parts coupe, it was a 3.73, it had the FD and M5 trans, go figure, it should have been a 3.54, but who knows the history of the car from dealer to me.
This is of course there is not something inherently different about P15 prodution in the USA.
I did not state in my post, that these ratios were specific to 1946-48 P15 6 cyl car., but were available for Chrysler built 6 cylinder cars, in this time period....