Thanks all for the good advice. I have been down the vacuum leak path and found many, including those you mentioned. I think they are plugged, after using lots of starter fluid to check. But Joe, you mentioned a vacuum line from the manifold. There is one which connects to the front Stromberg and actuates a throttle bypass valve at high vacuums. I had previously soaked those connections with starter fluid with no effect but decided to plug the line as you suggested. Made a significant improvement although I do not understand why. I had a large smile this afternoon on what might be my last test drive before the cursed winter arrives. As a plus, the now unused manifold tap provides a vacuum source which simplifies adding ignition advance.
David, as an aside, after removing the secondary butterflies, I was concerned that the spindle was then only held in place by friction and thought a vacuum leak might follow. Removed it also and plugged the holes. That may have been a waste of effort but my time is not worth that much. I also replaced the crossover pipe gaskets with aluminum plates and drilled a pressure relief hole in the pipe because having a large chamber connected to the intake manifold seemed worrisome.
Keith, you reinforce what SNG Barratt said about adding vacuum advance. They sell the unit quite reasonably and now that a vacuum connection is available on my manifold, it seem attractive.
So thanks again for the tips, now back to chasing rattles.