The center casting was unique to G bodies, because of the upper ears. B bodies also used upper ears, but at a different angle and location.
B body/earlier A body rears installed in a G body require modifications to the mounting system for the upper arms. Done well would use arms with a roto joint to accomodate the differences, and I beleive I have read of one installation with cut/welded upper arms.
Leaf spring/trucks didn't use mounting ears, so pretty unlikely it's one of those. Some (70-72) earlier A bodies used a few, they are wider than the G body unit. The "big" A bodies (73-77) also used them, and are even wider as I recall.
Might ask/look for the center section casting number (usually located at the front lower edge of the case), and check against known G body applications.
Like I said before, slim chance, but for the price legit g body 8.5s are bringing, I'd want to be sure what I was buying, But that's just me.