This note is an unsolicited testimonial for a carb shop that I had good luck with.
I own a 78 F250 with the 460 and the antichrist of the carburetor world, the Motorcraft 4350. About 4 years ago I had the engine rebuilt by a shop that turned out to be less than reputable. These guys claimed to be carburetor geniuses and could make that permanent flat spot go away. Instead what I got was a lopey idle like the truck had a race cam installed along with the flat spot.
After several attempts by these clowns to undo the damage with no better results, I then tried to do it myself with better results but still had the flat spot.

After an internet search I found a company called Pony Carburetors who advertised they had a cure for the 4350 blues. Since my other option was a manifold change and a new carb I decided to give them a try. They sent me a rebuilt 4350 exchange unit and I returned my old carb for core.
The truck runs perfectly. The carb has not been off the engine in 4 years and it still runs perfectly. I have no starting problems, no hesitations, no flat spots. My only complaint about these guys would be that it took a long time to receive core credit.
For those of you who still have this doorstop on their engine this might be a solution. (Be advised, they say if you have a '75 they may not be able to cure the problem. It evidently was the worst year.)