Problems with Holley 1920

You are pointing to the linkage of the "choke pull-off". Its purpose is to pop open the choke slightly as soon as the car starts. Review the manual. You set the opening to a drill bit size. The rubber diaphragms often leak because they don't like carb cleaner. Test it with a vacuum pump. A new one costs ~$10 if you can find one.

You can adjust the theromostat itself (rod coming in from the right). Remove it from the exhaust well and you will see rich (R) and lean (L) marks, where you rotate the spring tightness. The choke should be closed fully when the engine is cold, even if 70 F, but not real tight. Sounds like yours is set too tight. The old mechanical sensors like yours are hard to adjust well, and the choke stays on too long. If you can find a later electric-assist one for a slant, that would work better, but be sure to get the thermal sensor that goes with it. There was also an after-market version (thru J.C. Whitney), but it didn't work well for me on my 69 Slant years ago, but then I didn't understand the choke pull-off then either (mine was bad).