Thanks for the add. I have looked a fair amount about this topic and could not find a reasonable definitive answer. So I will post what I did.
I have a 1940 P10, 1 of the rear drums was cracked. I tried to price it reasonable but could not find anything under $200.
My rear plate to plate was 54” with 3” drum width. I went to the junk yard and got the rear out of a 2000 Jeep Cherokee- plate to plate - just about 53” with a drum width of 4”. I figured a 1/2” would not be a big deal. Turns out I was right. I cut the perches, and shock mounts off. Measured them out and put them on the other side during set up. My wheels bolted right up and I have a e-brake system to work with.
Just some info
Buddy