Although not a Slant six story I did have a valve get bent on a Fiat 1214 when while putting a new head on my "Helper " did not have the right side cam(OHC engine) in exactly the right spot and I did not catch it. Started it up and went to drive home and all of a sudden at about 35 MPH I heard a loud Bang, Bang, Bang. with that the engine seized. After pulling the head back off I found that the valve head had broken off from the stem and jammed the piston against the cylinder wall. So careful checking is a good idea.