1964-1972 Chevelles used different mounting locations than the full-size lineup. Of course, with the 1985 version of the Caprice, I really don't know. 80s G-bodies are similar in design to Chevelles (Mid-size), but still differ... IIRC, some companies like Currie, offer "Chevelle" brackets that you can weld to the 9-inch housing...
Same idea regarding the length. Generally, the Caprice being a "full size" car, the rearend will be longer than the "mid-size" Chevelle... Quick research indicates the 85 Caprice has a track width of 60.6 inches, while the 70 Chevelle is 59.9 inches... That's only 3/4 inch, but really 3/8" on each side... Not that much. That should be able to easily be overcome with wheel backspacing.
But depending on price and equipment (traction loc/positraction/locker/gear ratio) you may want to pick it up. 9-inch rears are relatively straight forward to narrow to the correct length.
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