In the 1970s when HD was on the skids (and AMF had just bought them), my shop (we also sold HD) kept getting new Sportsters that weeped oil through the side covers that were wetted, and the bottom. The one thing that finally worked was: after fully cleaning the engines, we ran them to hot and studied the leak spots, let it cool overnight, cleaned again, then finger-wiped aluminum-based paint onto/into those spots. We had already replaced cases and covers on some of these, only to get weeps in other places: the castings were porous. The aluminum paint method was still working 3 years (seasons) later when I left for Colorado. The method might help save this cylinder block?
All: using the 2.6mm thick O-rings for the CB750 head oil passages, even with a cylinder decked that has been milled back to flush with the oil passage recess, does not compress the O-ring far enough to block it, even with HD studs in place: the modern head gaskets are at least 1.0mm thick after torquing (some are 1.1mm thick, like some of the MLS 836cc gaskets I have dealt with). I never recommend anything for these bikes that hasn't been thoroughly measured, tested, and proven for years, unless it is my own ride. Then any mistakes can be my own fault.
