A 600 double pumper, with the same venturi diameters (AIRFLOW) as a 600 vacuum secondary (1-1/4" primary and 1-5/16" secondary), probably did not have the secondary opening link opening the secondaries fully, in order to generate less power. (Because) By the way, a "670" Street Avenger is a 600 vacuum secondary by it's venturi diameters... as well as a whole host of other troubles... all that I have seen (and remedied) have primary and secondary idle air bleeds that are too big, idle feed restriction sizes imbalanced and not specific for anybody's engine/vehicle combination, primary power valve channel restrictions too big, (<--- if at ~0.054", they attempt to compensate for) secondary jets that are too small, they frequently have metering blocks with idle mixture needle tip passage holes that are too big (0.100"+, original sizing meant for the blunt tip "reverse idle" mixture screws... making finding any sort of correct idle mixture next to impossible... standard idle (clockwise lean) mixture metering blocks require 0.063" passages for the sharp tip idle mixture screws), as well as air bleeds in the rear face of the primary metering block way up high in the "dog leg" of the main circuit air channels into the main wells (originally intended for metering blocks with "emulsion" tubes in the main wells... there haven't been "emulsion" tubes in main wells in Holley metering blocks in about 30 years...)... in a nutshell, I'm afraid I deem them another of the "new and improved" well-waxed expensive garbage of the utmost caliber.
You don't know how much camshaft is in it. How much idle vacuum will it presently generate? That'll give an idea on 0.050" intake duration, and subsequent correctly sized idle feed restrictions...
In general, you'd probably be miles ahead figuring out what's wrong with the 4776 and reusing it, or better yet, going with that 4777.
I don't have your cylinder heads, but my 306 is sporting most else yours is, including a bit over 9:1 and 218-degrees at 0.050 intake duration (she's bumpy enough idling at ~750rpm, and generates ~14-15"Hg vacuum), RPM intake with a 750 vacuum secondary, 14-degrees BTDC initial for starters (18 is possible (38 total mechanical) with a re-curved DuraSpark II distributor that's limited to provide only 10-degrees (distributor - 20 crankshaft degrees, "all in" at about 6000rpm... which is what a healthy small block Ford has always liked (it's the SBC's with their inefficient chambers that always needed the much-touted-by-magazines XX-degrees "all in by 2500rpm" nonsense... and with (the faster burning than the good (better than SBC) chambers SBF's have always had) combustion chambers like yours, you'd be best to re-curve the distributor to limit total mechanical advance to 32 or 34 degrees for best power))... it's not officially road-ready yet, but mine jalopy with 87-octane in the gas tank and one rip up the road and back and she's a real happy camper initially... super tuning to follow once road ready...
Additionally, after perusing all of the thread posts:
The blue (air:fuel ratio) line on the dyno sheet graph image of yours shows what I'm talking about above... the measuring of AFR starts off rich at 2500rpm @ 11:1 (matching the torque dip off the start), then dips richer yet at 3000rpm to 10:1, and then works it's way upward/lean (assuming secondaries open for sure by about 4000-4500rpm) and past 5000rpm it's gone onto the lean side of things @ 13:1 (producing the nervous looking, zig-zag saw blade-ing of the torque/hp lines). This is precisely how most "new and improved" carburetors that I have seen (and fixed) work... rich and mediocre at best everywhere down low and for acceleration and for cruise (lack of) fuel mileage, and then off into the most dangerous of danger zones for a carburetor to go lean, up top and at maximum operating rpm...
A correctly (and safely) calibrated and tuned carburetor will very quickly head straight to (richen, dip down to, from efficiency everywhere else like idle, low speed, light part throttle acceleration, cruise, etc.) a WOT AFR of about 12.5:1, and remain right at that AFR like a horizontal line all the way cleanly to the engine's red line rpm... most violent, smoothest torque and horsepower, bottom to top... with no danger then of blowing the engine to hell while fully loaded and at high rpm.
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