The backing plates for the 11.15" x 2 3/4" (JB5) have 3 different part #'s for the years '74 to '87:
74 to 75 .....331447, 331448
76 to 82 .....357853, 357854
83 t0 91 .....14055278, 14055279
I believe they will all bolt up to 10 and 12 bolt housings, but am not sure what other differences exist.
They use the same shoes and drums.
I do not see the JB5 option for '73.
Now the disclaimer: This information is the result of my research from GM parts manuals, and is in no way guaranteed to be
valid beyond the information contained in those publications.
I urge you do do your own research before investing in any of the above parts for your particular application.
(I feel much better now....)