JMH, Sorry you're still having trouble--she's give you a helluva time but look at the things you've learned through the battle.
Sounds like you've worked on the gasketed areas a bunch and are satisfied with their sealing ability.One way I have found leaks at the gasketed areas is to flood them with a water hose with the engine running.I know everybody wants to use carb cleaner or starting fluid but I'll try water flooding first if it's convenient.You can hear a difference if a leak is flooded.
Also consider valve guide condition.Not from a wear standpoint-that doesn't give your massive leak but I once had an engine totally destroy and pass a guide and it so affected the cylinder it shared a port with that we couldn't get that cylinder to hit either.Vacuum leak bigtime.
Maybe an inspection of inlet guides from above with some oil squirted around them might prove that's not a problem.
It just still sounds like that carb has a circuit blocked.
Please keep us posted.
