Don't bother trying to set up a progressive linkage for multiple carbs on a straight six. You are better off just having them all work simultaneously. The outer cylinders need to be fed just as much as the center cylinders. Given that the log manifold is a model of simplicity but also poor distribution, run multi carbs and don't make them too large.

I have a pair of brand new Carter-Weber staged 2 barrel carbs that I will run on my hot six. They have a single primary ventury and a single seconday venturi; that way you don't overdo it at low demand yet when you need it the carbs will breath. I bought mine through Stovebolt Engine Co, in Troy, Michigan. Linky: http://stoveboltengineco.com/

Here's a picture of my engine in progress: