1972 Pontiac Firebird engine was rebuilt

The gas will be stale and the engine won't run on it. If there isn't very much in the tank just add some fresh gas for now, and you'll likely have to dump a little down the carburetor and start the engine a few times. The oil will be okay but it will have run off the cylinder walls. The engine will likely crank a little sluggishly at first once you install a good battery. It may even appear to be locked up. If it won't crank at all, watch the headlights. If they get real dim when you try to start the engine, give up for now, remove all the spark plugs, and pour a little diesel fuel into each spark plug hole, then let it sit for a day or two. Diesel fuel has lubricating properties not found in gasoline and it will free up piston rings that are stuck to the cylinder walls.