Idle drops during warmup/stall!

Could it be icing?
At temps near zero, and high humidity,like Vancouver at this time of year, ice will/can form at the leading edges of the butterflies, when they are opened just a little ways as in cruising. Then when you come to a stop the ice shuts the fuel off at the low speed discharge ports and the engine stalls.
The cure is heat. Heat under the carb and/or to the throttle plates. A heavy dose of Isopropyl alchohol in the gas can help. There is a passage in the factory intake manifold,under the carb. It connects the two heads together.Hot exhaust shuttles back and forth under there and heats up the carb to prevent icing.As the motor ages and begins to burn oil, this passage often plugs up with carbon and the heat is no longer sufficient to prevent the icing.Perhaps that has happened to yours.
About the choke. Yes if you can adjust it to stay on longer it may help.
Another trick is to heat the air before it enters the air filter house.Later model 318s(early 70s?) had a heated air system which you should be able to adapt to your carb.It consisted of a different filter house with a snorkel on it, a metal plate to bolt onto the exhaust manifold, a connecting heat stove pipe between the two, and a vacuum control system that made it automatic.It works very well, supplying air at/near 80*F . The carb was calibrated for it as well.