[What'saCobra?]

It would be difficult to get a better performance/collar ratio than a new, out-of-box Holly 750 of the newest HP series. Choosing between mechanical (DP) or vacuum secondaries is simply a matter of personal choice, given that you are not racing. It is a feeling thing, rather than a speed thing.

i would suggest a 750 HP because you might make improvements in the future and might want the higher flow; say a new set of 304X FORD heads, or something. The above poster was correct about the original 715 cpfloat Holley for the 289 did work very well, both on the street and in racing.

The DP might require slight throttle feathering at the lowest rpm and WOT at low rpm might not be perfectly smooth. But, it will not be bad, since that is the specific correction that the newer HP series designs address. You might not even feel it, they are so newly sensitive to changes in airflow. The older Holleys did not really have a good fuel/air ratio control response at WOT and low rpm, because the slow moving mass air flow did not "suck" enough fuel into the airstream past the venturi because of old design airflow errors and not enough sensitivity at such slow velocities. Of course, the old designs were mostly fixable by good carb techies of the day for a cost.

But, when Demon really started to eat into Holley, the Holley R & D got hot and fixed what had been seemingly centuries old design flaws at WOT. Who of us did not have their carb go lean at WOT and low rpm when we used mech. secondaries on the street, especially?

The recent HP series corrects this and lots of other smaller but important effects, right out of the box.

Hard to recommend anything else to a non-techie, without taking a risk. But, good friends have also used the Demon and they recommend them. i have one on a rather hot FE from KC via our good friend Cracker, but have not yet run it, so i cannot say. But, it is a race engine anyway and the experience might not be at all useful.