A stud finder would be useless if you have strapping!
What style house is this? Your basic cape or ranch will most likely run front to back.
If I were you, I would cut a hole in a closet ceiling and check!
I would locate all wires and pipes before I started, as well.
NOTE, When I did this job I found a wire that had been cut and left between the walls, IT WAS HOT! (buy a wire tester to make sure you have the right breaker). Good luck,Jason
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