Carter Single BBS Issues

Do not bend the arm. On the underside of the choke thermostat there's a nut that can be loosened to rotate the spring assembly to apply greater or less tension to the pushrod, then tightened to lock the assembly in place. There's a big centre index mark, and smaller marks toward the letters R for richer (more tension) and L for leaner (less tension). The nominal adjustment is 2 notches towards R; if that's not where the unit you receive is set, set it there. Then bolt it onto the manifold and see how it does. If it doesn't push the choke all the way closed when you open the throttle with the engine cold, remove the choke thermostat from the manifold and move it a notch or two closer to R. You should not have to exceed a 4R adjustment. Keep in mind, the choke pull-off link also requires adjustment.
Hustle and get those three books quick as you can!

Carb base gasket, I am completely shocked OCPNW didn't have one for you—they probably do and just can't lay hands on it, that place is enormous. Anyhow, it's still available through ordinary parts supply lines. Walker number G850. If you strike out, send me a PM and I'll see if I have either a loose gasket (possibly) or a carb kit (almost certainly).