I would blame the converter for sure from what you describe. I had the same problem with a warmed over Chevy 230cu.in. 6 and Powerglide. Tried to get by with a stock converter and stalled every time I put it in gear. A 2000 stall converter solved my problem. A converter is one item that shouldn't be bought at a swap meet. There is no way to confirm the condition or it's properties.
Also, when you warm up an engine as you have, ignition becomes critical. If you don't have one, get an MSD module. You will be amazed how many sins it covers!