Ohh.....68's are out of my wheelhouse. I would think that hood clearance would be close to the same as a 69-72, but they are a slightly different animal and I really couldn't tell you.
What I would do is get some clay and make a ring around the outside of the air cleaner and put some on top of the nut and close your hood. If it doesn't touch then add more. When the hood makes contact you have your measurement of how much room you have to work with. Just don't forget about the flexing of the engine (unless you are running solid mounts) and you should be good to go.