Vacuum secondaries are usually better for economy, they don't dump as big of a fuel load when you hit the throttle.

Carb choice can be a nightmare. A 500 4 bbl seems to be a popular choice for the big sixes, but it is a little too big. I would lean toward dual carbs. A friend has dual Holley/Webers on his 261 that he built for performance. They really woke up the engine. Has good bottom end now, and has had it up to 6,000 rpm doing burnouts! Lots of power.

For some good advice on carbs, search this forum for posts by carbking, he's very knowledgable and does carb restoration for a living. He doesn't post here much anymore, but you can learn a lot from what he has written.

Fred
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