As for vacuum secondaries, I have never found a need for anything other than vacuum secondaries on the street. If you are considering the street demon it only comes with those.
You may have 'never found a need for anything other than vacuum secondaries on the street', but our favorite manufacturer (Mopar) used mechanical secondaries more than anything else. Only the later Holleys used vacuum. ALL of the Carters (AFBs, AVSs, Thermoquads) used mechanical secondaries. And that was on everything from 273s to Hemis.
Think Mopar might have known something you don't?
Most aftermarket carbs require some amount of mods for our Mopars. CarterBrocks will hit the choke stove on stock intakes, Holleys need the secondary springs dialed in, their Double Pumpers are pig rich on the street, all of them need adapters or fabrication on the linkage. It just comes with the territory. The Street Demons need some innovation on the linkage, too. And there are videos on Youtube to help with that. There are also clearance issues with the electric choke on some (not all) air cleaners. It is not tough, nor does it take anything like special skills or tools.
I have often used a Holley or CarterBrock Mopar throttle adapter on carbs other than what they were made for. A drill and some bits can work some real magic.
