rearend

Does anyone know off hand what was the standard gear ratio in the stock rearend of a 1976 Grand Prix with a 400 ci (auto-trans)?

there is no "standard" gear ratio. It all depends on how the car was ordered. It could have anything from a 2.73:1 to a 3.73:1 factory gear. If the factory tag is still on, the ratio will be stamped on it.