Uh Oh !! Bad knocking !!

In the old days we would put a piece of cigarette paper behind the bearing and it would take up the excess clearance. Not really suggesting you do this to 408, just saying it would work for a while. Not sure why you hope it is a head gasket. You may get off far easier if you just have to turn a journal. Last time I blew a head gasket between cylinders it either sucked or blew a piece of hourglass shaped gasket, metal and all into the cylinder. Block was beyond repair instantly almost. Piston too. Before you drive it any longer find out what it is, it can only get worse.