Installed new 0.038 idle tubes and 0.055 down channel restrictors after thoroughly cleaning out all passages in the carb, carefully reassembled and put the correct base gasket on. Set the idle screws at an initial 3-1/2 turns and like magic, I've got an engine that will idle now!
https://www.youtube.com/watch?v=WEAOsbrju2U
I think I can still back off on the choke a bit as it seemed to stay on the fast idle cam longer than necessary, and I'm tempted to drop down one number on the primary jets to see if I'm still running on the rich side - an A/F meter is definitely on the wish list.
It was nice to be able to finally enjoy a Saturday night drive in "The Bird". Thanks to all the tips from members and the great forum thread history that provided the idle tube and restrictor sizing Cliff had recommended to someone with a similarly cammed 400.