75 Duster: Six Pack or 4bbl?

That's a judgement call only you can make. I think you already know once the sixpack is tuned it is a blast and then the nostalgia of having it when you open the hood. :) My sixpack is fun for those reasons. Once mine was tuned it has not required anything for many years. It might come down to the cost for the rebuild of the carburetors and again only you can make that decision.