The top of the intake manifold, below the carburator base should be level (in Theory). When it is, the engine will look like it is installed on an angle, running "down" towards the rear of the car.----But think about it for a minute----if being on an angle was a real big concern, how the heck would we ever drive up or down hills???? If the car worked good with the 283 in it, then it will still be o.k. with a 350 in it without changing anything.