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Your axle should be the GM 8.5 inch 10-bolt.
It should hold up to the 400 horsepower no problem, but you will risk stuff with more power, slicks, and the like.
As far as the body goes I would look into aftermarket unibody bars that weld to the front and rear frame sections and also weld to the body in between.
You may want to look into some good measurements before doing so though, to make the car hasn't been too "tweaked" in the past.
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