"Soon to be a 383"...
Doing a converter now is a waste of money... That 350 will want a whole lot different stall then a 383. Wait till the 383 is done and you can talk to a converter manufacturer about your combo. They are going to need to know EVERYTHING... Carb, cam, compression, trans. gears, etc., etc.
With a mild 350 and that cam it shouldn't be idling at 1200 rpm... Do like Pat said and turn the idle down. Sounds like you need to do some tuning to get your current combo optimized.