[Excaliber]

You might not have needed to change jets either. Like I can run 68's AND an air cleaner and the car runs fine, all though under certain conditions it does emit the occasional blast of black smoke. Like if the idle falls to low for to long and it "loads up". Removing the air cleaners, with the 68's results in a slightly improved idle but about the same "feel" for accelleration, power, etc. It then never "loads up".

With 64 jets I HAVE to run the air filters to optomize the fuel/air ratio and get decent performance. Which tells me, that the TWO Stelling/Helling air filters are good for about a 4 jet size difference. A single Stelling/Helling with a single larger carb would no doubt require a different jet size change.

The question really becomes: Can a foam Stellings/Hellings air filter support 600 cfm? It's seems likely that it can, all though that may be nearing it's limit.

If you change TWO carbs by four points each on jet size is that the same as changing ONE carb by eight points? Assuming the single carb equals the same CFM as the two smaller dual carbs.