In your video the oil connector has the overflow pinched off. In 55 that should be a loop back down to the head. If you look at pre68 Dave's site, it clearly shows the different types of couplings between the rocker shafts. Maybe when you're running at highway speeds you are getting too much oil pressure in the rocker shafts. Thus, it's overwhelming the valve seals and guides. I would correct that coupling and see if it helps.
