A couple of suggestions ...
With a carb there's usually only one way to get the engine to idle high - and that is to have the throttle plate open too far - there are generally two ways to have the throttle plate open too far, the first is the idle stop screw, the second is the throttle cable - have you checked both of those?
It might be possible if you have a vacuum leak somewhere AND have a rich mixture that the engine won't lean out enough to stall because of the vacuum leak - that might also be worth checking since you are using a different carb.
One more suggestion - what sort of cold start mechanism does the carb have - if it's an auto choke - how does it disengage - water or electric, is it disengaging properly, is there a separate fast idle adjustment that may need resetting?