It’s possible, even likely, that the steel tube inside your Marvel heat riser jacket has rusted through. This causes a massive vacuum leak that is undetectable by normal procedures. Remove the intake manifold and look down the riser with a penlight and a pick looking for holes in the tube. If there aren’t any, there will be at some point in the future. The tubes used to be available but now I have to make them. I have done a bunch of these for Buick guys. It’s not a project for the average person. It takes equipment and a machinist.