408 Magnum New Build, Gutless of idle

Yeah, waaay too much cam for a stock ecm tune... Get it on the dyno and the ecm tuned for your combo, it'll make a biiig difference. And yes, the convertor could use some more stall. Stock convertors are for stock motors. 3.92s with a 33" tire isn't going to be ideal, but is workable. Did you actually degree the cam in, or take the factory at their word?