Success!!! I pulled the manifolds and found that the bottom edge of the exhaust manifold got caught on the top lip of the valve cover. So I had the width of the sheet metal valve cover leaving a gap at the bottom of the manifolds. That explains both the extra exhaust noise and the rough idle. After bending the valve cover lip out of the way and reinstalling the manifolds (and carburetor, fuel line, vacuum lines, throttle linkage....etc) it started up and purred like a kitten. Smooth as butter. For the time being I've adjusted the timing by ear, advancing it until it started to get a little rough, then retarding just a touch. Not very scientific, but I don't have a timing light right now. Now it is time to wrestle with the hood and try to get it lined up and springs attached. Ugh
One thing I did find out the hard way (like I learn most things) is that the studs go through to the water jacket. When I pulled the manifolds in April, one of the nuts was welded to the stud, probably from the heat of the exhaust manifold and 70 years of inattention. So the stud came out when I turned the nut. At the time it was not an issue, since all the fluids were drained in preparation for pulling the engine. When I put the manifolds back on two weeks ago I just used that nut/stud as if it were a bolt, not having a replacement stud on hand. Then when removing the manifolds again last week (after all the fluids were filled) I removed the nut/stud combination and got a steady stream of coolant. Live and learn. This time I ordered a replacement stud and used that when reassembling the manifolds.