One thing could be that air flow sensor plate is hanging up and that by removing the head and replacing it you some how dislodged the plate on its mount so it is no longer centered. There is a pivot arm and a disc that is held to the arm by a nut. These are really hard to see on B21s. (On a 75 the whole air system is easily accessible being located behind the passenger headlight).

If the plate gets knocked off center it will hang up on the venturi wall. to fix it you have to loosen the center nut and center the plate. easy to see and fix on a 75 --but not on the B21.

When you push it by hand is there an resistance????

The other thing could be that the rubber connector above the sensor plate- between it and the throttle body is not air tight---isn't there an O ring there around the top of air sensor throat sealing the bottom part of that rubber connector to the upper air sensor throat - where that big hose clamp is. If there is not a good seal the vacuum from the intake manifold would be "diluted" and not strong enough to pull up the plate.

(I know on the '75 set up--the snout bellows ( the one that runs from the air box on the B21)- this had to be solidly connected with no cracks and if it came off or loose the car wouldn't run -because on the 75 set up it was between the manifold - the vacuum source- and the air flow sensor plate.)

From your last description I'd look for a mechanical reason why the plate is not being raised by the airflow. Either a break in the vaccumm from the manifold to the air sensor plate, Or the plate itself being hung up. Start with verifying a tight seal.