The 80450 is listed as a 600 CFM square bore carburetor. This should break down as approximately 300 primary, 300 secondary.
The stock 7040264 is normally thought of as a 750 CFM spreadbore carburetor. This breaks down as approximately 150 primary, 600 secondary.
By using the stock manifold (150/600) with the square bore carb (300/300) you can figure basically the smaller of the two figures for both primary and secondary. You now have approximately 150 primary plus 300 secondary, or a 450 CFM carburetor/manifold combination.
Using the recommended Holley adapter will return some of the CFM, but causes the air/fuel mixture to make extra turns which an air/fuel mixture really doesn't like to do.
If you like Holleys, by all means use one; BUT change to the proper manifold. Personally, I think the 600 is a bit small for your combination. A 750 would be a better choice (again, change the manifold).
I would suggest you would be money and performance ahead by any of the following:
(A) acquiring the proper Q-Jet (spread-bore)
(B) having Cliff build a different Q-jet specific to your application
(C) using a Carter TQ aftermarket spread-bore (my personal choice)
Jon.
__________________
"Good carburetion is fuelish hot air". 
"The most expensive carburetor is the wrong one given to you by your neighbor".
If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!
Owner of The Carburetor Shop, LLC (of Missouri).
Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
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