Bud
The "matched" carbs from Summit/Jegs do not have anything special done to them. What Summit or Jegs mean by matched is they are the same part number
..and I'm being serious.
Plowman, I did not have to change the springs in my vacuum secondaries but in my friends street rod we did experiment with various springs. You mention you can run the vacuum secondaries by themselves(meaning without the vacuum balance kit). I found my car ran much better with the vacuum balance kit-because both secondaries open and close at the same time. I have yet to talk with someone who's engine ran worse with them. Of course everybodies setup is different...and some people just get lucky. I've talked to some people who took 450 holleys out of the box and they ran perfectly with no tuning whatsoever. Some people I talked with changed just about 3/4 of the components on the carbs and they still would not run right.
You talk about blocking metering plates etc to decrease fuel flow richness. I did that by running small 390's. I don't think my small 289 with metered down 600's would have run as good as the 390's.