Ive dealt with almost all of the Holley based OEM and OEM replacement Six packs, Tri -powers (GM speak), on both 340/440 and 427's, dated form 68-71.
WHat I have found out when it comes to the Holley replacement setups, is that the metering plates are not correct for the best running performance. Yes, they will run out of the box, but they are not set up like they were form back in the day form the factory: What makes his worse is that Holley has lost their technical savvy when t comes to these setups. They just dont really have anyone on on thier tech side that has the historical knowledge to get them right
Thats where your reputable six pack rebuilders/restorers come in. They can get the original carbs right by rebuilding them right, with the right parts. These type of guys know that even the Holley rebuild kits available do not have the correct parts in them for six pack setups. And some of these guys can take the new/OEM Six Pack replacements, and get them right
Then there are the throttle linkage setups/adjustments. I would conservatively say that 90% of the throttle linkages I have seen are not adjusted correctly. Close, but not right: And on the spot linkage adjustment is really key for optimal base performance out of these setups
And everytime the linkages are removed for any reason, (like removing a carb or two) they need to be adjusted again
2 Options
1. Try to find a used/OEM setup, and then have it rebuilt/restored by guys that know thier stuff. VERY expensive option
2. New, Holley six pack set ups: The new six packs from holley will run out of the box, and you could just run as is, or send the carbs to the folks that know their stuff to get them right (Maybe $300-$400)
Back in the day, because your average gear head didn't know how the sixes really worked, would convert the outboards to mechanical linkage openings and over the years, would just let the carbs go to hell, really because most average wrench folks just didn't know how to work with/on them
Most of your track racers used large 4 barrels instead of six pack setups. One reason was that they took too long to set up at the track when removed for quick work, as compared to a 4 barrel. Also it was easier to diagnose an issue with a 4 barrel than a six pack, essentially being three carbs (even though the outboards only came into play when vacuum demanded their use)
3,