Slant six dies in gear

Emma Godolphin said
I’ve run into a bit of a new problem now. Just a few days ago the replacement carb came in and we installed it with no issues. Made sure the jets and everything were clean. The car now refuses to idle below 25* and 1200rpm. Obviously it’s not drivable at that rpm so it’s at home right now. I’ve attempted to slow the rpm and lower the degrees by using the throttle every once in awhile to keep it running but no luck. I’ve had me look at it, and quite a few family members who are mechanics or heavy duty’s and we’re all stumped. I’m wondering if we caused an issue when we replaced the head. The head was from a 1975 slant six (we replaced it before we knew the engine was older and just before these issues began.) at this point the goal isn’t been running right but to get her to idle at all

I'm not that familiar with the slant six carbs and their replacements.
However, since this happened after changing the carb I would suspect the carb or the changes (vac hose rerouting or similar) that occured with the change.

Before going back to the previous carb, see if the reason for high idle rpm can be narrowed down.
For example, where is throttle positioned and what is holding it ? Is it on the slow idle stop or is the the throttle being held open on the fast idle cam's step?

Here's an illustration of the cam on a '73 BBD carb

Some photos of the current setup would help folks on this end see what versions/model of carb and emissions systems are on the engine. Some sharp eyes and knowledgable folks here.