hamonic balancer hitting
The best I can figure is that you have a 400 cast crank balancer and it bottoms out on the timing cover timing tab because a big block balancer snout is shorter than one for a small block. Without having a 360 and 400 crank side-by-side, it can be hard to tell. But a 400 big block balancer is thicker. Early 360 balancers had a counterweight while later ones had weight removed from the opposite side of the outer ring. But they were all about the same thickness, definitely thinner than what is on the engine.
Early 360 - Late 360 - 400 big block cast crank (I believed the last internet photo was labeled correctly.)
The car and engine are around 50 years old. It could have been rebuilt a few times, parts could have been mixed up and you can't always believe everything someone says. Check it yourself or find someone who knows and you can trust.
The best I can figure is that you have a 400 cast crank balancer and it bottoms out on the timing cover timing tab because a big block balancer snout is shorter than one for a small block. Without having a 360 and 400 crank side-by-side, it can be hard to tell. But a 400 big block balancer is thicker. Early 360 balancers had a counterweight while later ones had weight removed from the opposite side of the outer ring. But they were all about the same thickness, definitely thinner than what is on the engine.
Early 360 - Late 360 - 400 big block cast crank (I believed the last internet photo was labeled correctly.)