1977 Mercury Marquis stalls only in gear
 

Hello...i am hoping someone can lend their knowledge and insight to a problem I have been chasing. Recently bought a 1977 mercury marquis with 72k original miles. The issue im having is that the car will stall only while in gear and idling. Drives and runs smooth when its not stopped. Idles smooth when in neutral or park. Ive replaced the plugs, wires, cleaned out the carb and put in a new air filter. I did find a small vacuum leak and fixed that. The car is meant to be for my 17 year old daughter but need it to run better before I hand over the keys. Any help would be greatly appreciated.
400 motor, 2 barrel carburetor

Does the car stall or shut off in park? What transmission is in the car? Sounds like the converter may be locking up in the transmission. Have you checked the transmission fluid? I would start there. Could also be a vaccum leak in the carburetor, possibly throttle shaft bushings.