Here is a 37-41 spindle, referred to as a round back spindle.
and here are 42-48 spindles.
Functionally they are identical and both will go right on your model by using the kingpin set from the correct year group. 42-48 uses a slightly longer kingpin, but same diameter.
You can use 40-48 backing plates on either of the spindles, no difference whatsoever, except in 46-48 Ford went to a more simple 2 nut adjuster instead of the 4 nuts on the 40-45 backing plates. (actually, there were few cars made in 43-45 because of WWII.
The parts are so similar you could run a round back spindle on one side and a squareback on the other, or a early backing plate on one side and a later one on the other and it would make no difference. Except it would look goofy.
I have 40 brakes on one of my rods, and 46-48 on the other 2, and I prefer the later ones as you only have to play with two adjustment nuts, not 4.
Don
Oh, and some brake drums have the hub on the inside and some have the hub on the outside, but they all interchange. The ones with the hub on the outside are getting harder to find and are the ones you need if you want to modify Buick finned drums to run on the car.