You have a huge vacuum leak somewhere. you sure the big plug is on the back or front of the carb??
Spray Brake clean around the carb base then around the intake when the RPM picks up you have found your vacuum leak.
It could be under the intake if it is not sealing on the under side of the ports no good way to spray that like on a big block.

Pull the intake bolts a little tighter to see if it changes. could well be the end rail gaskets are holding intake up from sealing right. did you put those centering plastic things on the end rail gaskets may be helping to hold intake up.

BUT it is right there at the intake and carb area. If the carb is not being held open by a to tight throttle cable or other it's getting air somewhere else.

Pull the throttle cable and kick down off carb for now to make sure they are not holding carb part way open!

It's right there you will find it!

Long block cannot feed it the air to run up!! another vote.
It's carb held part open, intake to head seal, carb to intake seal or a big plug of intake or carb like where power brakes or PVC go.