my thoughts are that the block may not be perfect and the same for the cam. those bearings are babbit and i don't think there ever was a lot of clearance built into the design. after all who in 1956 on the design team would have ever thought we'd be subjecting these engines to the kind of forces a big roller cam can generate. anyhow, a thousandth out here, a thousandth out there and you can get the picture. the only saving grace is the soft bearing will conform;... to a point. I've used the "grooved" camshaft journal idea before but the truth is all it does is conform the bearings to the "grooved" cam and who knows if it's right. I also believe multiple camshaft changes on the same bearings can loosen things up too much.