Yeah hate to say it but your carb is on the HUGE side for the engine you described. I'd say hang on to it though and build a high compression 406 with a big solid roller cam and it will love it.
On the other hand if you just have to use that carb, first thing I would do is check your float level its idling to rich and has nothing to do with your main jets A likely cause could be a high float level after checking or adjusting float level and still idles rich you'll need to adjust your idle mixture screws, run them all the way in till lightly seated and back out one half turns, thats usually a good starting point then go either back in or out to get a smooth idle, remembering not to make to big a adjustments while doing this, some carbs are real senstive in this area. Goodluck, Charles