Should swap out fine. But by doing so, your rear tires will stick out past the fenders. Would be a great time to get some 1 1/4" adapters to put on the front hubs, and use some 4th gen rims instead.
Or you could just keep your current rear end, and just swap the brakes onto that, and swap out the differentials.
I believe you may also want to check if the rear end your wanting to get came from a car that had traction control. If it did, it will have 2 separate brake lines going to each rear caliper. In which case, you will have to cut off the end of the lines and put on a T fitting so the lines merge into one, so that you can hook it up to your current brake line. If the car didn't have traction control. It shouldn't be as difficult.
There are also some issues with the ABS setup. Here's a nice page with some good information on these issues:
http://www.funkz.net/firebird/rearendfaq.htm
If I were to guess, your current rear-end would be very similar to the first 2 rear-ends discussed on that page.
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