It wont fit directly. The Nova is a leaf spring car, the Caprice/Impala is trailing arm suspension. You would have to cut off all the mounts and weld on the right ones. Big pain in the ass if you don't know how to do that sort of thing. Also, the Impala diff is a lot wider so you would have to do some narrowing. With that comes finding axles, blah blah blah, and if you go that far, you might as well find a posi carrier and a good set of gears.
Your best bet would be to find a diff from another Nova or a first gen Camaro. 1968-74 Novas use the same floorpan, chassis, and cowl as a 1967-69 Camaro. Unfortunatly, those are going to be your only choices and even more unfortunate they are getting harder to find and more expensive to get parts for those cars.