75 Duster: Six Pack or 4bbl?

I would run with a 800 cfm TQ. Small primaries will give great response/mileage & huge secs will give HP.
There has also been improvements in intake manifold design since the days of the 6 pack.
If the engine is basically stock, low numerical gear ratio, I would use the Performer intake. If 'hotter' than stock, use the RPM intake.
An Edel 650 AVS would also be a good choice for a stock engine, 800 AVS for a hottie.
Three carbs look great, but 3 times as many things to cause problems....