the g body rear end is more than likely a 7.5 inch 10 bolt, when you say built what do you mean. they are not a strong axle, and you would be hard pressed to bring them up to the level of a stock nine inch.
off the top of my head the g-body axle will be skinnier than you impala, the g body is 57.75 inches from W.M.S to W.M.S. I don't know the specific width of the b-body, but im thinking its several inches wider. i believe that the upper control arm mounts will be different as well.
figure out what you have, its possible that its a larger, stronger axle than the one your looking at swapping in. or it may have the same internals as the "built" g body axle and you can swap the good stuff from the g body axle into yours.
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