6 pak run bad cold

A stock 440-6 should pull a hell of a lot more idle vacuum than that.

Did you actually adjust the idle mixure on the center carbs? IF so, "how did it act?" That is do the mixture screws seem to have normal effect? Are they "all out" or "all in?"

Could be a LOT of things. If you can't run it down, one thing I'd try is pull all the carbs and carefully inspect the gaskets and mating surfaces. Then figure a way to make block off's for the outer carbs. You could even cut some out of soda/ beer cans. Cut out a metal gasket from a soda can, but do NOT cut the throttle bore holes. If possible use an extra gasket, so manifold, gasket, block off, gasket, carb, on both outer carbs If you don't have, can't get extra base gaskets, just go manifold--base gasket--block off--carb.

This will at least allow you to twiddle the center carb and see if you can get it close.

I'd be lookin for leaks---vacuum leaks.

You have a shop manual? The sixpack setup is right in the service manual