On a Chevrolet there should be no machining required for the back cover to fit properly. Still, it is a good idea to check and make sure there is some clearance between the front of the engine and the back of the plate to allow oil drainback. This can be done with a couple pieces of clay. Place the clay at the highest points on the block and then press the cover into place. There was plenty of clearance on this block which has already had all the internals polished.