If you are running a stock distributor, you might be running into the same mechanical curve problem I had on my 340... with a mild cam, it needs a little higher idle speed and 15-18 deg advance at idle, and mechanical curve starts coming in around 900, adding 5-10 degrees advance, and when you drop it into drive, it loads the engine just enough to cause mechanical advance to drop off, and engine stalls.
I built an advance limiter plate with slots shortened on inside edge, so the advance springs were a little tighter at idle, yet limited total advance to 36 degrees all in. My advance curve now starts at 1300 instead of 900. With an auto, you should get about 200-250 RPM droop when put in D...
If you watch timing light and adjust idle speed from 600 to 900, if timing shifts from your initial, then you have same issue as I did... my timing was floating around 5 degrees and wouldn't stay the same on different days... kept fussing with it until I put in the limiter plate, then was rock solid...