I looked on the Rock and the pan gasket was a 55-56 year part. In 57 the # changed. I have a 265 block I will have to look at. All I can remember is that it does not have side mount provisions.
Jeff
Had a guy that I let use my shop to replace the engine in his 67 C/10. He had gotten an engine from his boss that was a spare or maybe the original for his 58 corvette. This guy came to me and said the block didn't have the side mount bolt bosses. He was showing how the bolt on mount would fit nice and flat to the side of the block and thought he could just chain the engine down tight agent's the mounts. I told him he could hook a chain to bumper of his truck and drag it out and take it home to do that. I wouldn't let work like that come out of my shop. He ended up getting a saddle mount for the front and bellhousing for side mounts.