im no carbie expert but i have an old holley troubleshooting book this is whats in there for your fault
Engine stalls when transmission is put into gear.
#)POSSIBLE CAUSE #) CORRECTION
1) Incorrect choke pull-off adjustment. 1) Readjust the pull-off or replace if necessary.
2) Fast idle RPM incorrect (too slow) 2) Speed up the fast idle to factory specs.
3) Engine running too lean because of vacuum leak or dirty jet. 3) Check for vacuum leak. Flow test carb to check jetting.