I'm not sure on a 68 but on my 66 there are Three ground straps under the hood.
Two of them go from the last valve cover bolt on the bottom,(one on each side) to the fire wall, and there is one on the frame to the inner fender.
If you are standing in front of the radiator facing the engine, it will be on your left side towards the front, If I were looking straight down from the corner of my battery tray thats where mine is, I couldn't imagine it being very different from my 66.
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