I have fitted a '56 open diff to my '40 , keeping the original 4spd box.
An adaptor/seal holder needed to be made for the rear of the 4spd. Not a big deal for someone with a lathe. And , of course , the front yoke needs to be modified to fit the 4spd splines etc.
I mounted the diff using the original spring center bolt . I moved the diff back a little till the front hole in the mount plate lined up ( there are 3 cavities in my original mount , the proper centering hole , and 2 casting relief holes ) . It is not a tight fit on the spring bolt , but is OK as it has been there for over 50,000 miles now.
