I have a 78 F-150 with a 460 C-6 and a 2.75 gearset. Back when it had a good engine in it, .040 over, zero decked, Reed 252/260 advertised with a 112 lobe sep, roller rockers, stock intake, an Edelbrock 750 leaned out on the cruise mode to just before it was surging. Had stock exhaust manifolds and dual 2 1/4 with flowmasters. The best mileage I got with the truck was 16.4 mpg's on flat highway at 60 mph towing an empty open car trailer. This mileage was not computed from my odometer. I topped off to the top of the filler neck, drove 100 miles going by mile markers and topped off again to the top of the filler neck. I was surprised at the mileage. I did the mile marker/top-off test several more times and came up with similar results.
Unfortunately later on the engine was severely overheated which started the downward spiral and one day I heard a noise which lasted for about 2 seconds and the end result is the #8 piston ended up everywhere else in the engine. I put in a stock 460 from a 78 Lincoln. With the same carb, intake and ignition settings the best I can get out of this engine is about 13. My feeling is the compression and .040 quench was a big factor in acheiving better mileage.
I have been really happy with the Edelbrock carbs. I've been running the same 750 on my F-150 for 9 years. I haven't touched it after the tuning was done----about 8 years ago. They are pretty much set-it and forget it.