I have a 1970 340-6 rebuilt to stock specifications. Originally the engine would not run properly, I was unable to idle it down below 1,000 rpm. Eventually I determined the carburetor metering plates on the end carburetors had been re-drilled as well as an incorrect metering block (generic) for the center carburetor. After several conversations with Holley and the Technical Department they assureded me the 2300 series carburetors they are manufacturing (4789, 4790, 4791, 4792) are manufactured to the same exact specs as was manufactured in 1970. The tech was able to pull a metering block assembly for my 4792 center carb off of the line (Holley P/N 34R6611-4AM, $73.17) along with 2 #41 machined metering plates (Holley P/N 34R6153-41, $30.98 ea.).

I'm not sure what your problem is with the carbs., but check your center carb base plate for any sloppiness whatsoever. The slightest amount of play in your center carb base plate throttle body will have a significant effect on the idle characteristics. Also, check that the correct jets are installed, (#61 auto, #63 manual) as both of these were a problem with my set up as well. Another thing to check are the air/fuel adjustment needles. When I purchased my metering block from from Holley it came with new air/fuel mixture screws installed but when I still couldn't get a consistent smooth idle, I checked the needles on my end carburetors. Looking under a magnifying glass it was obvious all four needles had been filed or slightly bent or in some other way not perfect. I ordered 4 new needles from Holley (P/N 15-3QFT, $4.98) and that was the last piece of the puzzle.

After making these changes and of course a complete rebuild kit installed, the engine ran perfectly, just like it did and was supposed to in 1970. Unfortunately, this was a 2-year process (2 winters) in diagnosing the problem, eliminating all other potential problems like a vacuum leak, distributor vacuum advance, fuel, etc. etc. I even had the carburetors professionally rebuilt (twice) and the problems detailed above were not caught. It seems they put a nice dichromate finish on the carbs,installed a new rebuild kit, but never really "dug into" quality of the parts that made up the carburetors...

I guess what I'm saying is that if you are doing a stock rebuild on a stock engine...I would get the parts described from the manufacturer of the carburetors designed for that engine.