I really favor a holley carb, personally, for its ease of tuning. However, for passenger cars that need a good low-end torque boost with good power when the secondaries kick open, I always liked the old Quadrajet too. I can see how it can be a deficit on a 22,000 pound vehicle however.
When you need a lot of airflow, I would favor a square bore Holley. You could use either a mechanical secondary or a vacuum secondary, however, for that large of a load, the vacuum secondaries would be opening up all the time anyway, so I don't know if there's any advantage to it. Usually, V-secondaries are a little better at fuel economy, but they say go with a M-secondary on a manual transmission.
Another option I don't see mentioned much here is the Barry Grant line of carbs. They resemble Holley carbs and even use some Holley parts, but are not Holley carbs. They used to be known as the Silver Claw and the Gold Claw, but are now sold under the Demon line of carbs.
What makes a BG carb different is that it has 4-corner idling and you set the jets on all 4-corners the same, whereas on a Holley, the secondaries are usually about 10 steps richer than the primaries. It's more of a 4-barrel, with all 4-barrels doing the work, instead of 2 barrels with a pair of secondaries. Like a full-time 4-bbl, if you will.
You might consider trying a BG Demon carb. Might work best for you. Just be careful not to tune it like a Holley. Keep the above tip in mind.
