I would start with the intake manifold alignment rings, one of them is not seated properly causing a big vacum leak. If not that keep looking for vacum leak, though I think if you just disconnect vacum hose from the intake that should just about eliminate all source of vacum from the system. Then you can check carb and intake better.
Otherwise check pushrods and vavle settings, then timming. Good luck, let us know what you find.
