Large cams or cams that act large, like yours, can cause some plug issues. There's a lot going on in the chamber at a low idle. I can't recommend a particular plug, but a step or two may help with the low end idle and performance.
The other issue regarding the converter.......Is that your current stall or from the previous combo? I can only think that the new replacement that has yet to be installed will make some difference in the character of how it idles.