Need recommendation for a Carb for better MPG!

OK, I get it. My expectations were probably a bit absurd.
You expectations are not absurd. I'm slightly amused that people answering you, don't know how to get 20+ mpg from a 340. You are in Nebraska, nice mid-western state with relatively flat terrain, highway driving, 55-60 mph (takes a lot of aerodynamic drag out of the picture), 2,200 rpm @ 65 mph sounds about right for 3.23 gears. What do you know about the 340 in your car? Do you have a timing light? Is your rear an 8 3/4?

So much bad info in this thread... Are you running good solid core wires? What ignition and distributor are you running? Platinum plugs? Anyway, start with a timing light and disconnect the vacuum advance from the distributor and plug it. Where is your timing set at now? Bump it up 2 degrees at a time, reconnect your vacuum advance tube, and check mpg on your normal drive. That is the starting point. Then, you can start swapping mixture rods in the Edelbrock carb, same scientific steps going richer and leaner in small steps.

I have gotten mid 20's (25 mpg average) mpg around 70 mph in a 4 speed, 1964 Barracuda, 2.76 gears running very steady (that's where the vacuum gage comes into play) and with the lightest pedal pressure that will maintain speed, with 72 340 heads, the stock 340 cam, and "71" 340 intake and TQ on a high compression 273 short block. I've done almost as good with a stock "72" 340. So much for "the 340 cam will never give good mpg" pronouncement.

The TQ, Quadrajet, and Street Demon have triple boosters and very small primary throttle bores, which have the potential of running very well and getting good mpg. That said, every time you accelerate, even the slightest amount, you are pumping more fuel into the engine. The TQ is IMHO the best street carb ever made. Like everything made, if abused or misused they can have problems. I still run 50 year old TQ's on my "68" 383, "73" 318 and the "67" 273. No problems yet with probably 400,000 on the 3 combined.