My first guess would have been secondary butterflies not opening.
How do you have the ignition timed? If it is retarded it will adversely affect everything
as well as make it prone to overheating, especially the exhaust.
If plugging the vacuum ports makes it better it is obviously either a vacuum leak or
carbs set too lean.
Make sure your manifold nuts are good and tight and the manifold gasket is intact.
Are you sure you have a good flow of fuel from the fuel pump?
Disconnect the hose from the carbs and run it into a can and watch while someone cranks it.
You should get a pretty strong, intermittent stream every other rotation of the engine.
If you had a stuck valve your compression reading on that cylinder would be zero or near zero.
If none of the above helps, next steps are making sure the exhaust is clear and then
checking valve lift. All valves should open at least 3/8" measured at the valve spring retainer.
If they don't, your cam has gone south.
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George Downs Bartlesville, Oklahoma