Just bought a smoke machine.

I'm not sure if these 460 motors had the AIR pump or not but if it does I would check the tube that bolts to the back of the heads. I have had plenty of 5.0 and 5.8 motors that had either the check valve rot away or the tube itself burn out, causing what sounds exactly like an exhaust leak.
My solution is to remove the pipe and cut the ends off, then crimp them shut and bolt back on as plugs for the holes, remove the AIR pump and related crap, and install an idler pulley. Some also have a small tube running to the catalytic converter, guaranteed to rot off.