Dan,
I know the magnums all use the narrower top ears and require about .350 spacer on the single offset ear to use the 318 bracket. If it was a BB mount, it would be offset inward and have a steel tube welded. Just odd. It looks like an A-body mount, but the mount offset is backwards. The guy who sold them to me on ForBbodiesonly said they were off a '69 318. I assumed a B-body for obvious reasons and the fact that the drivers side is right for a B-body.I'll just drill the hole I need once it is sitting in the K-frame.