When you say Sunny pick ups, I assume you have a b120,b122 or b140 utility/bakkie, not a b310, which is the model known as the Sunny here in Australia. The S12 diff can be made to fit, but all the trailing arm mounts would need to be remove, & the standard Sunny(b120) mounts welded on. If you measure the widths, the S12 diff is about 100 mm/4 inches wider than the B310 diff,which is wider than the b120 by 50mm/2 inches. So the rear brakes would be well outside the rear wheel arches. You could narrow the S12 diff Housing & axles, because you've already got it & the diff & axles are strong enough. As any alternative you could get the H190 diff housing from a C120 Vanette that had the A15, they have a 4.1:1 steel centre, are leaf spring already,although they have the same size axles as the H165.
Also,where abouts are you from? Not being rude or anything, but it always helps to know what advice to give
you. You might not have the model I've recommended to you easily available, it might have a different name, & it all helps to give you information you can use. Please fill out your profile.Thanks.