Another one on the its not the long block side here! Intake manifold is probably leaking. Either its not torqued correctly, gaskets are no good, goobered up too much rtv around a port or something, it wont line up correctly, etc.

You did use new gaskets, right?

Did you take the carb off of the manifold or did you swing the entire thing as a unit? Check to make sure that you did not disturb the choke.

PCV port plugged in or blocked? That's an easy one to miss because its in the back. Maybe old dry rotted vacuum caps that you disturbed when you swing the manifold and carb?

Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street