Keep in mind that the factory rear end wasn't designed to handle the torque from a high horsepower engine. You're going to have to change rear ends if you plan on " jumping " on the power band of a high performance engine.
I had a factory 4-speed, 1981 El Camino that blew out several rear ends when I applied real torque thru the engine. They just blow out. The best part is that they're available in every wrecking yard ( G bodies are out there however, the right gears are hard to find )and you'll become an expert at changing rear ends!! Look for either a Hurst 442 or a Buick Gran National to swap your rear end, otherwise, you too will become an expert in changing rear ends.