I suggest you head for
CB750 gas tanks at EBay and go over the pictures. I'm confident there will be a few stock gas tanks, and hopefully they'll have the factory decals on them.
Good luck, and I hope this helps.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.