[Anthony]

I think for a car with a standard transmission, with a lot of shifting gears, especially with short time intervals between shifts, as the car accelerates well, you would be better off with mechanical secondaries.

The reason why holley double pumpers got bad gas mileage is I believe because they came standard with "richer" jetting than factory carbs, and therefore would get worse gas mileage with cruising, as the richer jetting was better for performance. Rejet them lean like factory carbs, and the fuel mileage will be the same.

A vacuum secondary carb has a slight delay in the secondaries opening. You can play with different vacuum springs, to the get the weakest spring that will not cause the motor to bog, but it still will not open as fast as a double pumper. Why I say this is that between every shift, if you let up on the throttle, both the carb primaries and secondaries close, and when you step back on the gas, the secondaries are again slightly delayed in opening. With an automatic transmission, you mash the throttle in first, and never lift up on the throttle between shifts, so to me, it doesn't matter as much in this case. But, if you're like me, and lift up on the throttle with every shift, I think you would be better with a double pumper.

One final thing, as you are using a dual plane intake, likely with a divided plenum, I would at least go with a 750cfm carb, if not higher, 780-850. If you run an open plenum intake, you could probably do fine with a 700-750 cfm.