1966 Plymouth Valiant /6 & Holley 1945 carb can only get to idle at 1250rpm. How to get a lower idle without dying?

Yup, what he ^^ said. The dead giveaway is that it runs better when the choke is closed. That says it’s starving for fuel. Likely the main circuit has trash in it.
Make sure you use some compressed air after soaking your carb ! Blowout every hole you see , from both ends of the passage ! You probably already know this , but just saying !