Brake booster check valve grommet Vacuum Leak - Stalling

The airleak at the booster grommet may not be 100% of your stalling.
What's your idle-timing?
If you run too much idle-timing, the throttle will be too far closed at idle, and the transfers will slow right down. Depending on your cam-size, the Transfers could even dry up. Then when you put it in gear the sudden load application will cause a sudden loss of airspeed past the transfers/Idle discharge ports, and thus the engine stalls.