I disagree that a square bore carb can't do as well as a spread.

I've tuned a 625 road demon on my 440 winni. The 440 is original, well worn, and burns some oil.
I get 8 mpg (us gallon) and 10mpg Cdn gallon towing my car over some of the highest mountain passes in the world.

Car's 3800pounds, plus the 18 ft trailer, the tools, and of course the winni with the gear and the family in it, which I'm scared to even guess at the weight on. It get's better milleage than any gas tow rig I've ever owned. TQ or otherwise.

But back to the build....

I agree that the dp is going to hurt mileage, BUT it should only be a factor when your romping on it. In the primary's it shouldn't matter.

I notice no cam specs. A 340 was a performance motor, so it wants to perform. Not economize.

IMHO I think the 13 mpusg you are getting isn't too far off of anything you can hope for.

If there is something wrong with the holley then you may see some improvement.

A smaller carb (not smaller jets) may improve velocity, and fuel atomization, so if you can find something smaller (regardless of brand) you may see an improvement there also.

Put a vacuum gauge on it, and watch it while you drive. Your driving habits may adjust a bit and help you too.

It's very difficult to build an old motor to perform, and then get mileage out of it.