I'm not sure of the actual ID of those connecting tubes, but I would guess 1/4"?
IMHO no actual transfer of air occurs through them (between adjacent carburetors), but they function as a buffer for vacuum pulses at very low engine speeds.
If you're modifying Stromberg 81, 97 etc. remember that they almost certainly have the same jetting etc. in both sides - but they don't have to if you convert them to progressive. Since each primary is small the jetting can be lean for good mileage without reducing response, and the secondary can be rich for maximum power without loss.
I believe the 48 is the largest (common) model that fits the 3-bolt flange, it's about 180 CFM.
NO, do not use the [CFM = D X VE X RPM / 3,456] to get the correct size!!
