Engines will often idle fine but have problems when in gear. When going into gear, there is a load on the engine so it needs to make more power to handle that. I would suspect your timing is a bit too retarded so you might want to loosen the distributor and move it slightly counter clockwise. That will raise the idle speed some so you might want to adjust that. If that helps, you might want to move the distributor a little more until it no longer stalls going into gear. All it takes is a little change like this to make your engine happy.